|
|
5 ............... 1506 459.02
6 ............... 1540 469.38
7 ............... 1504 458.41
8 ............... 1600 487.67
9 ............... 1640 499.86
10 ............... 1645 501.39
These figures show that this lake exceeds in depth the deepest
of the Swiss lakes (the Lake of Geneva), which has a maximum
depth of 334 meters. On the Italian side of the Alps, however,
Lakes Maggiore and Como are said to have depths respectively
of 796.43 and 586.73 meters. These two lakes are so little
elevated above the sea that their bottoms are depressed 587
and 374 meters below the level of the Mediterranean.
(2.) _Relation of Temperature to Depth_. By means of
a self-registering thermometer (Six's) secured to the
sounding-line, a great number of observations were made on the
temperature of the water of the Lake at various depths and
in different portions of the same. These experiments were
executed between the 11th and 18th of August, 1873. The same
general results were obtained in all parts of the Lake. The
following table contains the abstract of the average results,
after correcting the thermometric indications by comparison
with a standard thermometer:
Obs. in Feet in Meters F. deg. in C._
1 ...... 0-Surface 0-Surface 67 19.44
2 ...... 50 15.24 63 17.22
3 ...... 100 30.48 55 12.78
4 ...... 150 45.72 50 10.00
5 ...... 200 60.96 48 8.89
6 ...... 250 76.20 47 8.33
7 ...... 300 91.44 46 7.78
8 ...... 330 (Bottom) 100.58 45.5 7.50
9 ...... 400 121.92 45 7.22
10 ...... 480 (Bottom) 146.30 44.5 6.94
11 ...... 500 152.40 44 6.67
12 ...... 600 182.88 43 6.11
13 ...... 772 (Bottom) 235.30 41 5.00
14 ...... 1506 (Bottom) 459.02 39.2 4.00
It will be seen from the foregoing numbers that the
temperature of the water decreases with increasing depth to
about 700 or 800 feet (213 or 244 meters), and below this
depth it remains sensibly the same down to 1506 feet (459
meters). This constant temperature which prevails at all
depths below say 250 meters is about 4 degrees Cent. (39.2
Fah.). This is precisely what might have been expected; for it
is a well established physical property of fresh water,
that it attains its maximum density at the above-indicated
temperature. In other words, a mass of fresh water at the
temperature of 4 deg. Cent. has a greater weight under a
given volume (that is, a cubic unit of it is heavier at this
temperature) than it is at any temperature either higher or
lower. Hence, when the ice-cold water of the snow-fed streams
of spring and summer reaches the Lake, it naturally tends to
sink as soon as its temperature rises to 4 deg. Cent.; and,
conversely, when winter sets in, as soon as the summer-heated
surface water is cooled to 4 deg., it tends to sink. Any
further rise of temperature of the surface water during the
warm season, or fall of temperature during the cold season,
alike produces expansion, and thus causes it to float on
the heavier water below; so that water at 4 deg. Cent.,
perpetually remains at the bottom, while the varying
temperature of the seasons and the penetration of the solar
heat only influence a surface stratum of about 250 meters in
thickness. It is evident that the continual outflow of water
from its shallow outlet cannot disturb the mass of liquid
occupying the deeper portions of the Lake. It thus results
that the temperature of the surface stratum of such bodies of
fresh water for a certain depth fluctuates with the climate
and with the seasons; but at the bottom of deep lakes it
undergoes little or no change throughout the year, and
approaches to that which corresponds to the maximum density of
fresh water.
(3.) _Why the Water does not freeze in Winter_. Residents
on the shore of Lake Tahoe testify that, with the exception
of shallow and detached portions, the water of the Lake never
freezes in the coldest winters. During the winter months, the
temperature of atmosphere about this Lake must fall as low,
probably, as 0 degrees Fah. (-17.78 deg. Cent.). According to
the observations of Dr. George M. Bourne, the
minimum temperature recorded during the winter of 1873-74 was
6 deg. Fah. (-14.44 deg. Cent.). As it is evident that during
the winter season the temperature of the air must frequently
remain for days, and perhaps weeks, far below the freezing
point of water, the fact that the water of the Lake does not
congeal has been regarded as an anomalous phenomenon. Some
persons imagine that this may be due to the existence of
subaqueous hot springs in the bed of the Lake--an opinion
which may seem to be fortified by the fact that hot springs do
occur at the northern extremity of the Lake. But there is
no evidence that the temperature of any considerable body of
water in the Lake is sensibly increased by such springs. Even
in the immediate vicinity of the hot springs (which have in
summer a maximum temperature of 55 deg. C. or 131 F.), the
supply of warm water is so limited that it exercises no
appreciable influence on the temperature of that portion of
the Lake. This is further corroborated by the fact that no
local fogs hang over this or any other portion of the Lake
during the winter which would most certainly be the case if
any considerable body of hot water found its way into the
Lake.
The true explanation of the phenomenon may, doubtless, be
found in the high specific heat of water, the great depth of
the Lake, and in the agitation of its waters by the strong
winds of winter. In relation to the influence of depth, it
is sufficient to remark that, before the conditions preceding
congelation can obtain, the whole mass of water--embracing a
stratum of 250 meters in thickness--must be cooled down to
4 deg. Cent.; for this must occur before the vertical
circulation is arrested and the colder water floats on the
surface. In consequence of the great specific heat of
water, to cool such a mass of the liquid through an average
temperature of 8 deg. Cent, requires a long time, and the cold
weather is over before it is accomplished. In the shallower
portions, the surface of the water may reach the temperature
of congelation, but the agitations due to the action of
strong winds soon breaks up the thin pellicle of ice, which is
quickly melted by the heat generated by the mechanical action
of the waves. Nevertheless, in shallow and detached portions
of the Lake, which are sheltered from the action of winds and
waves--as in Emerald Bay--ice several inches in thickness is
sometimes formed.
[Illustration: Lily Lake]
[Illustration: Cave Rock, Lake Tahoe]
[Illustration: Pyramid Peak and Lake of the Woods]
[Illustration: Clouds Over the Mountain, Lake Tahoe]
(4.) _Why Bodies of the Drowned do not Rise_. A number of
persons have been drowned in Lake Tahoe--some fourteen
between 1860 and 1874--and it is the uniform testimony of the
residents, that in no case, where the accident occurred in
deep water, were the bodies ever recovered. This striking fact
has caused wonder-seekers to propound the most extraordinary
theories to account for it. Thus one of them says, "The water
of the Lake is purity itself, but on account of the highly
rarified state of the air it is not very buoyant, and swimmers
find some little fatigue; or, in other words, they are
compelled to keep swimming all the time they are in the water;
and objects which float easily in other water sink here like
lead." Again he says, "Not a thing ever floats on the surface
of this Lake, save and except the boats which ply upon it."
It is scarcely necessary to remark that it is impossible that
the diminution of atmospheric pressure, due to an elevation
of 6250 feet (1905 meters) above the sea-level, could sensibly
affect the density of the water. In fact, the coefficient of
compressibility of this liquid is so small that the withdrawal
of the above indicated amount of pressure (about one-fifth
of an atmosphere) would not lower its density more than one
hundred-thousandth part! The truth is, that the specific
gravity is not lower than that of any other fresh water of
equal purity and corresponding temperature. It is not less
buoyant nor more difficult to swim in than any other fresh
water; and consequently the fact that the bodies of the
drowned do not rise to the surface cannot be accounted for by
ascribing marvelous properties to its waters.
The distribution of temperature with depth affords a natural
and satisfactory explanation of the phenomenon, and renders
entirely superfluous any assumption of extraordinary lightness
in the water. The true reason why the bodies of the drowned
do not rise to the surface is evidently owing to the fact that
when they sink into water which is only 4 deg. Cent. (7.2
deg. Fah.) above the freezing temperature, the gases usually
generated by decomposition are not produced in the intestines;
in other words, at this low temperature the
bodies do not become inflated, and therefore do not rise to
the surface. The same phenomenon would doubtless occur in
any other body of fresh water under similar physical
conditions.[2]
[Footnote 2: It should be noted that since 1874 there have been
remarkably few deaths from drowning in Lake Tahoe, and that the major
cases of those referred to by Dr. LeConte were of workmen and others
who were generally under the influence of intoxicants.]
(5.) _Transparency of the Water_. All visitors to this
beautiful Lake are struck with the extraordinary transparency
of the water. At a depth of 15 to 20 meters (49.21 to 65.62
feet), every object on the bottom--on a calm sunny day--is
seen with the greatest distinctness. On the 6th of September,
1873, the writer executed a series of experiments with the
view of testing the transparency of the water. A number of
other experiments were made August 28 and 29, under
less favorable conditions. By securing a white object of
considerable size--a horizontally adjusted dinner-plate
about 9.5 inches in diameter--to the sounding-line, it
was ascertained that (at noon) it was plainly visible at a
vertical depth of 33 meters, or 108.27 English feet. It must
be recollected that the light reaching the eye from such
submerged objects must have traversed a thickness of water
equal to at least twice the measured depth; in the above
case, it must have been at least 66 meters, or 216.54 feet.
Furthermore, when it is considered that the amount of
light regularly reflected from such a surface as that of a
dinner-plate, under large angles of incidence in relation
to the surface, is known to be a very small fraction of
the incident beam (probably not exceeding three or four per
cent.), it is evident that solar light must penetrate to
vastly greater depths in these pellucid waters.
Moreover, it is quite certain that if the experiments
in relation to the depths corresponding to the limit of
visibility of the submerged white disk had been executed in
winter instead of summer, much larger numbers would have been
obtained. For it is now well ascertained, by means of the
researches of Dr. F.A. Forel of Lausanne, that the waters of
Alpine lakes are decidedly more transparent in winter than in
summer. Indeed, it is reasonable that when the affluents of
such lakes are locked in the icy fetters of winter, much less
suspended matter is carried into them than in summer, when all
the sub-glacial streams are in active operation.
Professor Le Conte goes into this subject (as he later does into the
subject of the color of Lake Tahoe) somewhat exhaustively in a purely
scientific manner and in too great length for the purposes of this
chapter, hence the scientific or curious reader is referred to the
original articles for further information and discussion.
_Color of the Waters of Lake Tahoe_. One of the most
striking features of this charming mountain Lake is the
beautiful hues presented by its pellucid waters. On a calm,
clear, sunny day, wherever the depth is not less than from
fifty to sixty meters, to an observer floating above its
surface, the water assumes various shades of blue; from a
brilliant Cyan blue (greenish-blue) to the most magnificent
ultramarine blue or deep indigo blue. The shades of blue
increasing in darkness in the order of the colors of the
solar spectrum, are as follows: Cyan-blue (greenish blue),
Prussian-blue, Cobalt-blue, genuine ultramarine-blue, and
artificial ultramarine-blue (violet blue). While traversing
one portion of the Lake in a steamer, a lady endowed with a
remarkable natural appreciation and discrimination of shades
of color declared that the exact tint of the water at this
point was "Marie-Louise blue."
The waters of this Lake exhibit the most brilliant blueness
in the deep portions, which are remote from the fouling
influences of the sediment-bearing affluents, and the washings
of the shores. On a bright and calm day, when viewed in the
distance, it had the ultramarine hue; but when looked fair
down upon, it was of almost inky blackness--a solid dark
blue qualified by a trace of purple or violet. Under these
favorable conditions, the appearance presented was not unlike
that of the liquid in a vast natural dyeing-vat.
A clouded state of the sky, as was to be expected, produced
the well-known effects due to the diminished intensity of
light; the shades of blue became darker, and, in extreme
cases, almost black-blue. According to our observations,
the obscurations of the sky by the interposition of clouds
produced no other modifications of tints than those due to
a diminution of luminosity.
In places where the depth is comparatively small and the
bottom is visibly white, the water assumes various shades
of green; from a delicate apple-green to the most exquisite
emerald-green. Near the southern and western shores of the
Lake, the white, sandy bottom brings out the green tints very
strikingly. In the charming _cul-de-sac_ called "Emerald
Bay," it is remarkably conspicuous and exquisitely beautiful.
In places where the stratum of water covering white portions
of the bottom is only a few meters in thickness, the green hue
is not perceptible, unless viewed from such a distance that
the rays of light emitted obliquely from the white surface
have traversed a considerable thickness of the liquid before
reaching the eye of the observer.
The experiments with the submerged white dinner-plate,
in testing the transparency of the water, incidentally
manifested, to some extent, the influence of depth on the
color of the water. The white disk presented a bluish-green
tint at the depth of from nine to twelve meters; at about
fifteen meters it assumed a greenish-blue hue, and the blue
element increased in distinctness with augmenting depth,
until the disk became invisible or undistinguishable in the
surrounding mass of blue waters. The water intervening between
the white disk and the observer did not present the brilliant
and vivid green tint which characterized that which is seen in
the shallow portions of the Lake, where the bottom is white.
But this is not surprising, when we consider the small amount
of diffused light which can reach the eye from so limited a
surface of diffusion.
In studying the chromatic tints of these waters, a hollow
pasteboard cylinder, five or six centimeters in diameter, and
sixty or seventy centimeters in length, was sometimes employed
for the purpose of excluding the surface reflection and the
disturbances due to the small ripples on the water. When
quietly floating in a small row-boat, one end of this
exploring tube was plunged under the water, and the eye of
the observer at the other extremity received the rays of light
emanating from the deeper portions of the liquid. The light
thus reaching the eye presented essentially the same
variety of tints in the various portions of the Lake as those
which have been previously indicated.
Hence it appears that under various condition--such as depth,
purity, state of sky and color of bottom--the waters of this
Lake manifest nearly all the chromatic tints presented in
the solar spectrum between greenish-yellow and the darkest
ultramarine-blue, bordering upon black-blue.
It is well known that the waters of oceans and seas exhibit
similar gradations of chromatic hues in certain regions.
Navigators have been struck with the variety and richness of
tints presented, in certain portions, by the waters of the
Mediterranean Sea, the Atlantic and Pacific Oceans, and
especially those of the Caribbean Sea. In some regions of the
oceans and seas, the green hues, and particularly those tinged
with yellow, are observed in comparatively deep waters, or, at
least, where the depths are sufficiently great to prevent
the bottom from being visible. But this phenomenon seems to
require the presence of a considerable amount of suspended
matter in the water. In no portion of Lake Tahoe did I observe
any of the green tints, except where the light-colored bottom
was visible. This was, probably, owing to the circumstance
that no considerable quantity of suspended matter existed in
any of the waters observed.
_Rhythmical Variations of Level in Lakes: or
"Seiches."_--As might be expected, the waters of Lake
Tahoe are subject to fluctuations of level, depending upon
the variable supplies furnished by its numerous affluents. In
mid-winter, when these streams are bound in icy fetters, the
level falls; while in the months of May and June, when the
snows of the amphitheater of mountain-slopes are melting most
rapidly, the level of the Lake rises, and a maximum amount
of water escapes through its outlet. According to the
observations of Capt. John McKinney, made at his residence
on the western shore of this Lake, the average seasonal
fluctuation of level is about 0.61 of a meter; but in extreme
seasons it sometimes amounts to 1.37 meters. The Lake of
Geneva, in like manner, is liable to fluctuations of level
amounting to from 1.95 to 2.60 meters, from the melting of the
Alpine snows.
But besides these variations of level due to the variable
quantities of water discharged into them by their affluents,
many lakes of moderate dimensions are liable to rhythmical
oscillations of level of short duration, which are, obviously,
but produced by fluctuations in the supply of water. It is to
this kind of species of variation of level that our attention
will be directed in the sequel.
This interesting phenomenon was first recognized in the Lake
of Geneva; but was subsequently found to be common to all
the Swiss lakes, as well as to those of Scotland. It is,
therefore, a general phenomenon, which may be observed in all
lakes of moderate dimensions. The inhabitants of the shores
of the Lake of Geneva have long designated this rhythmical
oscillation of the level of the water by the term of
_Seiche_; and this designation has been adopted by
scientific writers.
These _Seiches_ were first signalized in the Lake of
Geneva in 1730, by Fatio de Duillier, who ascribed them to the
checking of the flow of the waters of the Rhone on the shoal
near Geneva by the force of the wind at mid-day. Addison
and Jallabert, in 1742, supposed them to be caused by sudden
increments in the discharge of the affluents, due to the
augmentation in the amount of snow melted after mid-day; or
to the sudden increase in the flow of the Arve, checking
the outflow of water by the Rhone. Bertrand supposed that
electrified clouds might locally attract and elevate the
waters of the lake, and thus produce oscillations of level.
H.B. de Saussure, in 1799, attributed the phenomenon to rapid
local variations of atmospheric pressure on different parts
of the lake. J.P.E. Vaucher, in 1802 and 1804, adopted de
Saussure's explanation, and confirmed it by many excellent
observations. He, moreover, established that _Seiches_,
more or less considerable, occur in all the Swiss lakes; and
that they take place at all seasons of the year, and at all
times of the day; but, in general, more frequently in spring
and autumn. As regards the cause of the phenomenon, Vaucher
shows how rapid local alterations of atmospheric pressure
would produce oscillations in the level of the lake, and
compares them to the vibrations of a liquid in a recurved tube
or siphon. Finally, Arago maintained that _Seiches_ may
arise from various causes, and traced the analogy between them
and certain remarkable oscillations
of the sea, including those arising from earthquakes.
But physical science is indebted to Professor F.A. Forel, of
Lausanne, for the most complete and exhaustive investigation
in relation to the phenomena of _Seiches_. This
accomplished physicist began his researches in 1869, and has
continued them up to the present time. He has been able to
demonstrate that these rhythmical oscillations occur in nearly
all the Swiss Lakes (he studied the phenomena in nine of
them), and that they follow in all cases the same general
laws. Those of the Lake of Geneva have received the most
elaborate and prolonged investigation. In March, 1876, Forel
established a self-registering tide-gauge (_limni-metre
enregistreur_) on the northern shore of this lake, at
Morges; and, with the coöperation of P. Plantamour, another
one was installed in June, 1877, at Secheron, near the city
of Geneva, at the southern extremity. Since these dates,
these two instruments have, respectively, been registering
oscillations of the level of the water of the Lake of Geneva;
and they are so sensitive as to indicate the waves generated
by a steamer navigating the lake at a distance of ten or
fifteen kilometers.
From a most searching investigation of all the phenomena
presented by the _Seiches_ in the Swiss Lakes, Forel
deduces the conclusion that they are really movements of
steady uninodal oscillations (balanced undulations), in which
the whole mass of water in the lake rhythmically swings
from shore to shore. And, moreover, he shows that the water
oscillates according to the two principal dimensions of the
lake; thus, giving rise to longitudinal _Seiches_
and transverse _Seiches_. They occur in series of
tautochronous oscillations of decreasing amplitude; the first
wave produced by the action of a given cause having a maximum
amplitude.
_Causes_. The disturbances of hydrostatic equilibrium
which generate _Seiches_ may be produced by a variety of
causes. Among these, the following may be cited: (a) Sudden
local variations of atmospheric pressure on different parts of
the lake. (b) A descending wind, striking the surface of the
lake over a limited area, (c) Thunder-storms, hail-storms, and
water-spouts; and especially when the accompanying winds act
vertically. (d) The fall of a large avalanche, or of a
land-slide into the lake. (e) And lastly, earthquakes.
Observations show that the most frequent and evident of
these causes are variations of atmospheric pressure and local
storms. With regard to earthquake shocks as a cause of such
fluctuations of level, it is a singular and significant fact
that since Forel has established the delicate self-registering
apparatus on the shores of the Lake of Geneva, no less than
twelve earthquake shocks have been experienced in this portion
of Switzerland, and they have had no sensible influence on
these sensitive instruments. In fact, a little consideration
in relation to the character of such shocks renders it highly
improbable that such brief tremors of the earth's crust
could have been any agency in the generation of rhythmical
oscillations of the whole mass of water in the lake. Indeed,
it is very questionable whether any earthquake waves are ever
produced in the ocean, except when the sea-bottom undergoes a
permanent vertical displacement.
_Lake Tahoe_. From inquiries made of the inhabitants of
the shores of Lake Tahoe, I was not able to discover that any
rhythmical oscillations of the level of its waters have ever
been noticed. Some residents declared that they had observed
sudden fluctuations of level, which, from their suddenness,
they were disposed to ascribe to disturbances of the bottom of
the Lake due to volcanic agencies, although they were
unable to coordinate such oscillations with any earthquake
manifestations on the adjacent shores.
It is evident, however, that until arrangements are
consummated for recording systematic observations on the
variations of the level of this Lake, we cannot expect
that its _Seiches_ will be detected. Of course,
self-registering gauges would give the most satisfactory
results; but any graduated gauge, systematically observed,
would soon furnish evidence of the phenomenon. For the
longitudinal _Seiches_, "Hot Springs," at the northern
extremity of the Lake, or "Lake House," at the southern end,
would be eligible stations for gauges; and for the transverse
_Seiches_, Glenbrook, on the eastern shore, or Capt.
McKinney's on the western margin, would afford good stations.
As far as I am aware, true _Seiches_ have never been
observed in any of the American lakes. This fact is the more
remarkable from the circumstance that long-continued and
careful observations have been made on the fluctuations of
level of several of the large Canadian lakes, with the view of
testing the possible existence of lunar tides. Perhaps these
lakes may be too large to manifest the uninodal rhythmical
oscillations which have been so successfully studied by Forel
in the smaller lakes of Switzerland.[3]
Be this as it may, there can be no doubt that Lake Tahoe is a
body of water in all respects adapted for the manifestation of
this species of oscillation; and that, like the Swiss lakes,
it is subject to _Seiches_. Indeed, the far greater
simplicity in the configuration of the basin of Lake Tahoe
than that of the Lake of Geneva must render the phenomena much
less complicated in the former than in the latter.
Professor LeConte then gives his computations as to the probable
duration of the oscillations on Lake Tahoe, should they occur there.
[Footnote 3: It is proper to add that _Fluctuations of level in the
North American lakes_ have been noticed by various observers, from
the time of the Jesuit Fathers of the period of Marquette, in 1673,
down to the present epoch. Among those who have discussed this problem
may be mentioned in chronological order: Fra Marquette in 1673, Baron
La Hontan 1689, Charlevoix 1721, Carver 1766, Weld 1796, Major S.A.
Storrow 1817, Capt. Henry Whiting 1819, H.R. Schoolcraft 1820, Gen.
Dearborn 1826-29.]
CHAPTER VII
HOW LAKE TAHOE WAS FORMED
Lindgren, the geologist, affirms that after the Sierra Nevada range
was thrust up, high into the heavens, vast and long continued
erosion "planed down this range to a surface of comparatively gentle
topography." He claims that it must originally have been of great
height. Traces of this eroded range (Cretaceous) "still remain in
a number of flat-topped hills and ridges that rise above the later
tertiary surface. There is reason to believe that this planed-down
mountain range had a symmetrical structure, for somewhat to the east
of the present divide is a well-marked old crest line extending from
the Grizzly Peak Mountains on the north, in Plumas County, at least
as far south as Pyramid Peak, in Eldorado County. At sometime in
the later part of the Cretaceous period the first breaks took place,
changing the structure of the range from symmetrical to monoclinal and
outlining the present form of the Sierra Nevada."
This great disturbance he thinks, "was of a two-fold character,
consisting of the lifting up of a large area including at least a
part of the present Great Basin [Nevada and Utah] and a simultaneous
breaking and settling of the higher portions of the arch. Along the
eastern margin a system of fractures was thus outlined which toward
the close of the Tertiary was to be still further emphasized. The main
break probably extended from a point south of Mono Lake to Antelope
Valley and from Markleeville northward toward Sierra Valley. A large
part of the crust block to the west of this dislocation also sank
down. This sunken area is now indicated by Lake Tahoe and by its
northward continuation, Sierra Valley, separated from each other only
by masses of Tertiary lavas.... It is worthy of note that within the
area of the range no volcanic eruptions accompanied this subsidence."
He continues: "As a consequence of this uplift the erosive power of
the streams was rejuvenated, the Cretaceous surface of gentle outline
was dissected, and the rivers began to cut back behind the old divide,
carrying their heads nearly to the present crest line that separates
the slope of the Sierra from the depression of Lake Tahoe."
These rivers are the great gold bearing streams that caused the mining
excitement of 1849. They all head near the Tahoe region, and include
the Yuba, Feather, American, Mokelumne, Calaveras, Cataract, and
Tuolumne.
Here, then, were two crest lines--the old Cretaceous line of which the
Crystal Range immediately overlooking Desolation Valley on the west,
with Pyramid and Agassiz Peaks as its salient points,--and the new
Tertiary crest line, reaching somewhat irregularly from Honey Lake
in the north to Mono Lake in the south. At the north of Lake Tahoe,
"southwest of Reno, a large andesitic volcano poured forth lavas which
extend between the Truckee River Canyon and the Washoe Valley. In the
region extending northward from Lake Tahoe to Sierra Valley enormous
andesitic eruptions took place, and the products of these volcanoes
are now piled up as high mountains, among which Mount Pluto nearly
attains 9000 feet."
These are the volcanic lavas which united the two crests forming the
eastern and western borders of the Tahoe basin or depression, and
through which the Truckee River had in some way to find passage ere it
could discharge its waters into Pyramid Lake, resting in the bosom of
the Great Basin.
Here, then, we have the crude Tahoe basin ready for the reception
of water. This came from the snow and rainfall on its large and
mountainous drainage area, a hundred greater and lesser streams
directly and indirectly discharging their flow into its tremendous
gulf.
Its later topography has been materially modified by glacial action,
and this is fully discussed by Professor Joseph Le Conte in the
following chapter.
It should not be forgotten, however, that while Mt. Pluto was being
formed, other vast volcanic outpourings were taking place. Well back
to the west of the Tahoe region great volcanoes poured out rhyolite,
a massive rock of light gray to pink color and of fine grain, which
shows small crystals of quartz and sanidine in a streaky and glossy
ground mass. On the summits nearer to Tahoe the volcanic outflows were
of andesite, a rough and porous rock of dark gray to dark brown color.
Lindgren says: "By far the greater part of the andesite occurs in the
form of a tuffaceous breccia in numerous superimposed flows. These
breccias must have issued from fissures near the summit of the range
and were, either before their eruption or at the time of issue, mixed
with enormous quantities of water, forming mud flows sufficiently
fluid to spread down the slope for distances of fifty or sixty
miles. The derivation of the water and the exact mode of eruption are
difficult to determine.... Towards the summits the breccias gradually
lose their stratified character and become more firmly cemented. Over
large areas in the Truckee quadrangle the andesite masses consist of
breccias containing numerous dykes and necks of massive andesite....
"The andesite volcanoes were mainly located along the crest of the
Sierra, in fact, almost continuously from Thompson Peak, west of Honey
Lake, down to latitude 38° degrees 10'. Farther south the eruptions
diminished greatly in intensity.... Along the first summit of the
range west of Tahoe the greatest number of vents are found. Beginning
at Webber Lake on the north, they include Mount Lola, Castle Peak,
Mount Lincoln, Tinker Knob, Mount Mildred and Twin Peak. The andesite
masses here in places attain a thickness of 2000 feet. An interval
followed in the northern part of the Pyramid Peak quadrangle where no
important volcanoes were located, but they appear again in full force
in Alpine County. Round Top, attaining an elevation of 10,430 feet,
and the adjacent peaks, were the sources of the enormous flows which
covered a large part of Eldorado County. Still another volcanic
complex with many eruptive vents is that situated in the western part
of Alpine County, near Markleeville, which culminates in Highland Peak
and Raymond Peak, the former almost reaching 11,000 feet. The total
thickness of the volcanic flows in this locality is as much as 4000
feet."
It is to these breccias we owe the volcanic appearances in the Truckee
River Canyon, a few miles before reaching the Lake. There are several
layers of the andesites breccias at the head of Bear Creek Canyon,
above Deer Park Springs.
"None of the craters," says Lindgren, "of these volcanoes are
preserved, and at the time of their greatest activity they may have
reached a height of several thousand feet above the present summits."
CHAPTER VIII
THE GLACIAL HISTORY OF LAKE TAHOE
We have already seen in the preceding chapter how the great basin,
in which Lake Tahoe rests, was turned out in the rough from Nature's
workshop. It must now be smoothed down, its angularities removed, its
sharpest features eliminated, and soft and fertile banks prepared upon
which trees, shrubs, plants and flowers might spring forth to give
beauty to an otherwise naked and barren scene.
It is almost impossible for one to picture the Tahoe basin at this
time. There may have been water in it, or there may not. All the
great mountain peaks, most of them, perhaps, much higher by several
thousands of feet than at present, were rude, rough, jagged masses,
fresh from the factory of God. There was not a tree, not a shrub, not
a flower, not a blade of grass. No bird sang its cheering song, or
delighted the eye with its gorgeous plumage; not even a frog croaked,
a cicada rattled, or a serpent hissed. All was barren desolation,
fearful silence and ghastly newness.
What were the forces that produced so marvelous a change?
Snowflakes,--"flowers of the air",--as John Muir so poetically calls
them. They accomplished the work. Falling alone they could have done
nothing, but coming down in vast numbers, day after day, they piled
up and became a power. Snow forms glaciers, and glaciers are mighty
forces that create things.
[Illustration: Gilmore Lake, Pyramid Peak and the Crystal Range,
in winter, from summit of Mount Tallac]
[Illustration: Desolation Valley, Looking Toward Mosquito Pass]
[Illustration: Heather Lake, near Glen Alpine]
[Illustration: Susie Lake, near Glen Alpine Springs]
Let us, if possible, stand and watch the Master Workman
doing the work that is to make this region our source of present
day joy. We will make the ascent and stand on the summit
of Pyramid Peak. This is now 10,020 feet above sea level,
rising almost sheer above Desolation Valley immediately at our feet.
The first thing that arrests the visitor's attention is the peculiar
shape of the peak upon which he stands, and of the whole of the
Crystal Range. Both east and west it is a great precipice, with a
razor-like edge, which seems to have been especially designed for
the purpose of arresting the clouds and snow blown over the mountain,
ranges of the High Sierras, and preventing their contents falling upon
the waste and thirsty, almost desert-areas of western Nevada, which
lie a few miles further east.
Whence do the rains and snow-storms come?
One hundred and fifty miles, a trifle more or less, to the westward
is the vast bosom of the Pacific Ocean. Its warm current is constantly
kissed by the fervid sun and its water allured, in the shape of mist
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