snow-clad summits would form a more effectual separation between
the plains on either side than would the widest ocean, were it not
that transverse valleys are of frequent occurrence, which open up a
pass, or way of transit, at a level below the snow-line. But even
these would not prevent the range being an impassable barrier, if
the temperate regions contained as lofty mountains as the tropics.
Mountain ranges, however, decrease in height from the equator to the
poles in relation to the snow-line.
The numerous attempts that have been made to generalize on the
distribution of mountains on the globe have hitherto been almost
unsuccessful. In America, the mountains take a general direction more
or less parallel to the meridian, and for a distance of 8,280 miles,
from Patagonia to the Arctic Ocean, form a vast and precipitous range
of lofty mountains, which follow the coast-line in South America,
and spread somewhat out in North America, presenting everywhere
throughout their course a tendency to separate into two or more
parallel ridges, and giving to the whole continent the character of
a precipitous and lofty western border, gradually lowering into an
immense expanse of eastern lowlands. In the Old World, on the other
hand, there is no single well-defined continuous chain connected
with the coast-line. The principal ranges are grouped together in a
Y-shaped form, the general direction of which is at right angles to
the New World chain. The centre of the system in the Himalayas is
the highest land in the hemisphere. From this, one arm radiates in
a northeast direction, and terminates in the high land at Behring
Strait: the other two take a westerly course; the one a little to the
north, through the Caucasus, Carpathians, and Alps, to the Pyrenees;
the other more to the south, through the immense chain of Central
African mountains, and terminating at Sierra Leone. Most of the
principal secondary ranges have generally a direction more or less at
right angles to this great mountain tract.
The inquiry into the origin of mountains is one that has received
not a little attention. Geologists have shown that the principal
agents in altering the surface of the globe are denudation, which is
always abrading and carrying to a lower level the exposed surfaces,
and an internal force which is rising or depressing the existing
strata, or bringing unstratified rocks to the surface. Whether the
changes are the small and almost imperceptible alterations now taking
place, or those recorded in the mighty mountains and deep valleys
everywhere existing, denudation and internal force are the great
producing causes. These give us two great classes of mountains.
The extent to which denudation has altered the surface of the globe
can scarcely be imagined. All the stratified rocks are produced by
its action; but these do not measure its full amount, for many of
these beds have been deposited and denuded, not once or twice, but
repeatedly, before they reach their present state. Masses of rock
more indurated, or better defended from the wasting currents than
those around, serve as indices of the extent of denudation. The most
remarkable case of this kind with which we are acquainted is that of
the three insulated mountains in Ross-shire--Suil Veinn, Coul Beg,
and Coul More--which are about 3,000 feet high. The strata of the
mountains are horizontal, like the courses of masonry in a pyramid,
and their deep red color is in striking contrast with the cold bluish
hue of the gneiss which forms the plain, and on whose upturned edges
the mountain-beds rest. It seems very probable, as Hugh Miller
suggests, that when the formation of which these are relics (at one
time considered as Old Red Sandstone, but now determined by Sir
Roderick Murchison as being older than Silurian) was first raised
above the waves, it covered with an amazing thickness the whole
surface of the Highlands of Scotland, from Ben Lomond to the Maiden
Paps of Caithness, but that subsequent denudation swept it all away,
except in circumscribed districts, and in detached localities like
these pyramidal hills.
Mountains produced by internal force are of several kinds. (_a_)
Mountains of ejection, in which the internal force is confined to a
point, so to speak, having the means of exhausting itself through an
opening in the surface. The lava, scoriæ, and stones ejected at this
opening form a conical projection which, at least on the surface,
is composed of strata sloping away from the crater. Volcanoes are
mostly isolated conical hills, yet they chiefly occur in a somewhat
tortuous linear series, on the mainland and islands which inclose
the great Pacific Ocean. Vesuvius and the other European volcanoes
are unconnected with this immense volcanic tract. (_b_) But the
internal force may be diffused under a large tract or zone, which, if
it obtain no relief from an opening, will be elevated in the mass.
When the upheaval occurs to any extent, the strata are subjected to
great tension. If they can bear it, a soft rounded mountain-chain
is the result; but generally one or more series of cracks are
formed, and into them igneous rocks are pushed, which, rising up
into mountain-chains, elevate the stratified rocks on their flanks,
and perhaps as parallel ridges. Thus, the Andes consist of the
stratified rocks of various ages, lying in order on the granite and
porphyry of which the mass of the range is composed.
The position of the strata on such mountains supplies the means of
determining, within definite limits, the period of upheaval. The
newest strata that have been elevated on the sides of the mountain
when it was formed, give a date antecedent to that at which the
elevation took place, while the horizontal strata at the base
of the mountains supply one subsequent to that event. Thus, the
principal chain of the Alps was raised during the period between the
deposition of the Tertiary and that of the older recent deposits.
(_c_) But there is yet another way in which the upheaving internal
force operates, viz., where it does not act at right angles to the
surface, but rather obliquely, and, as it were, pushes the solid
strata forward, causing them to rise in huge folds, which, becoming
permanent, form parallel ranges of mountains.
The crust of the earth, in its present solid and brittle condition,
is thus curved, in a greater or less degree, by the shock of every
earthquake; it is well known that the trembling of the earth is
produced by the progress of a wave of the solid crust; that the
destruction of buildings is caused by the undulation; and that the
wave has been so evident that it has been described as producing
a sickening feeling on the observer, as if the land were but thin
ice heaving over water. The Appalachians were thus formed. Many
other ranges have had a similar origin, as some in Belgium and in
the Southern Highlands of Scotland, as has been suggested by Mr.
Carruthers.
It is evident that in the last two classes the parallel ridges
were produced at the same time. Elie de Beaumont generalized this,
maintaining that all parallel ridges or fissures are synchronous;
and on this he based a system of mountain structure which is too
universal and too geometrical to be true. The synchronism of parallel
fissures had been noticed by Werner, and it is now received as a
first principle in mining. The converse is also held to be generally
true, that fissures differing in direction differ also in age;
yet divergence from a centre, and consequent want of parallelism,
as in the case of volcanoes, may be an essential characteristic
of contemporaneity. Nevertheless, Elie de Beaumont classified the
mountains of the world according to this parallelism, holding that
the various groups are synchronous. The parallelism does not consist
in having the same relations to the points of the compass--for these,
as regards north and south, would be far from parallel--but is
estimated in its relation to some imaginary great circle, which being
drawn round the globe would divide it into equal hemispheres. Such
circles he calls Great Circles of Reference. But beyond this, he went
a step further, and proposed a more refined classification, depending
on a principle of geometrical symmetry, which he believed he had
discovered among his great circles of reference. It is to be feared,
however, that his geometrical speculations have little foundation in
nature.
LAKES--FRESH, SALT, AND BITTER
--SIR ARCHIBALD GEIKIE
Depressions filled with water on the surface of the land, and known
as Lakes, occur abundantly in the northern parts of both hemispheres,
and more sparingly, but often of large size, in warmer latitudes. For
the most part, they do not belong to the normal system of erosion in
which running water is the prime agent, and to which the excavation
of valleys and ravines must be attributed. On the contrary, they are
exceptional to that system, for the constant tendency of running
water is to fill them up. Their origin, therefore, must be sought
among some of the other geological processes.
Lakes are conveniently classed as fresh or salt. Those which possess
an outlet contain in almost all cases fresh water; those which have
none are usually salt.
In the northern parts of Europe and America, as first emphasized by
Sir Andrew C. Ramsay, lakes are prodigiously abundant on ice-worn
rock-surfaces, irrespective of dominant lines of drainage. They
seem to be distributed as it were at random, being found now on the
summits of ridges, now on the sides of hills, and now over broad
plains. They lie for the most part in rock-basins, but many of them
have barriers of detritus. In the mountainous regions of temperate
and polar latitudes, lakes abound in valleys, and are connected with
main drainage-lines. In North America and in Equatorial Africa, vast
sheets of fresh water occur in depressions of the land, and are
rather inland seas than lakes.
The water of many lakes has been observed to rise above its normal
level for a few minutes or for more than an hour, then to descend
beneath that level, and to continue this vibration for some time. In
the Lake of Geneva, where these movements, locally known there as
_Seiches_, have long been noticed, the amplitude of the oscillation
ranges up to a metre or even sometimes to two metres. These
disturbances may sometimes be due to subterranean movements; but
probably they are mainly the effect of atmospheric perturbations,
and, in particular, of local storms with a vertical descending
movement.
Among the geological functions discharged by lakes the following may
be noticed:
1st. Lakes equalize the temperature of the localities in which they
lie, preventing it from falling as much in winter and rising as much
in summer as it would otherwise do.[1] The mean annual temperature of
the surface water at the outflow of the Lake of Geneva is nearly 4°
warmer than that of the air.
2d. Lakes regulate the drainage of the area below their outfall,
thereby preventing or lessening the destructive effects of floods.
3d. Lakes filter river-water and permit the undisturbed accumulation
of new deposits, which in some modern cases may cover thousands
of square miles of surface, and may attain a thickness of nearly
3,000 feet (Lake Superior has an area of 32,000 square miles;
Lago Maggiore is 2,800 feet deep). How thoroughly lakes can filter
river-water is typically displayed by the contrast between the muddy
river which flows in at the head of the Lake of Geneva, and the
“blue rushing of the arrowy Rhone,” which escapes at the foot. The
mouths of small brooks entering lakes afford excellent materials
for studying the behavior of silt-bearing streams when they reach
still water. Each rivulet may be observed pushing forward its delta
composed of successive sloping layers of sediment. On a shelving
bank, the coarser detritus may repose directly upon the solid rock of
the district. But as it advances into the lake, it may come to rest
upon some older lacustrine deposit. The river Linth since 1860 has
annually discharged into Lake Wallenstadt some 62,000 cubic metres of
detritus.
A river which flows through a succession of lakes can not carry much
sediment to the sea, unless it has a long course to run after it has
passed the lowest lake, and receives one or more muddy tributaries.
Let us suppose, for example, that, in a hilly region, a stream passes
through a series of lakes. As the highest lake will intercept much,
perhaps all, of this sediment, the next in succession will receive
little or none until the first is either filled up or has been
drained by the cutting of a gorge through the intervening rock. The
same process will be repeated until the lakes are effaced, and their
places are taken by alluvial meadows. Examples of this sequence of
events are of frequent occurrence in Britain.
Besides the detrital accumulations due to the influx of streams,
there are some which may properly be regarded as the work of lakes
themselves. Even on small sheets of water, the eroding influence
of wind-waves may be observed; but on large lakes the wind throws
the water into waves which almost rival those of the ocean in size
and destructive power. Beaches, sand-dunes, shore-cliffs, and other
familiar features of the meeting-line between land and sea, reappear
along the margins of such great fresh-water seas as Lake Superior.
Beneath the level of the water a terrace or platform is formed, of
which the distance from shore and depth vary with the energy of the
waves by which it is produced. This platform is well developed in the
Lake of Geneva.
Some of the distinctive features of the erosion and deposition that
take place in lake-basins have been admirably laid open for study in
those basins of vanished lakes which have been so well described by
Gilbert, Dutton, Russell, and Upham in the Western Territories of the
United States. They have been treated of in a masterly way by Gilbert
in his essay on _The Topographic Features of Lake-Shores_.
4th. Lakes serve as basins in which chemical deposits may take place.
Of these the most interesting and extensive are those of iron-ore,
which chiefly occur in northern latitudes.
5th. Lakes furnish an abode for a lacustrine fauna and flora,
receive the remains of the plants and animals washed down from
the surrounding country, and entomb these organisms in the
growing deposits, so as to preserve a record of the lacustrine
and terrestrial life of the period during which they continue.
Besides the more familiar pond-snails and fishes, lakes possess a
peculiar pelagic fauna, consisting in large measure of entomostracous
crustaceans, distinguished more especially by their transparency.
These, as well as the organisms of shallower water, doubtless furnish
calcareous materials for the mud or marl of the lake bottoms. But
it is as receptacles of sediment from the land, and as localities
for the preservation of a portion of the terrestrial fauna and
flora, that lakes present their chief interest to a geologist. Their
deposits consist of alternations of sand, silt, mud, gravel, and
occasional irregular seams of vegetable matter, together with layers
of calcareous marl formed of lacustrine shells, _Entomostraca_, etc.
In lakes receiving much sediment, little or no marl can accumulate
during the time when sediment is being deposited. In small, clear,
and not very deep lakes, on the other hand, where there is little
sediment, or where it only comes occasionally at intervals of flood,
thick beds of white marl, formed entirely of organic remains, may
gather on the bottom, as has happened in numerous districts of
Scotland and Ireland. The fresh-water limestones and clays of some
old lake-basins (those of Miocene time in Auvergne and Switzerland,
and of Eocene age in Wyoming, for example) cover areas occasionally
hundreds of square miles in extent, and attain a thickness of
hundreds, sometimes even thousands, of feet.
Existing lakes are of geologically recent origin. Their disappearance
is continually in progress by infilling and erosion. Besides the
displacement of their water by alluvial accumulations, they are
lowered and eventually drained by the cutting down of the barrier at
their outlets. Where they are effaced merely by erosion, it must be
an excessively slow process, owing to the filtered character of the
water; but where it is performed by the retrocession of a waterfall
at the head of an advancing gorge, it may be relatively rapid after
it has once begun. In a river-course it is usual to find a lake-like
expansion of alluvial land above each gorge. These plains may be
regarded as old lake-bottoms, which have been drained by the cutting
out of the ravines. Successive terraces often fringe a lake and mark
former levels of its waters. It is when we reflect upon the continued
operation of the agencies which tend to efface them, that we can best
realize why the lakes now extant must necessarily be of comparatively
modern date.
Saline lakes, considered chemically, may be grouped as _salt lakes_,
where the chief constituents are sodium and magnesium chlorides
with magnesium and calcium sulphates; and _bitter lakes_, which are
usually distinguished by their large percentage of sodium carbonate
as well as chloride and sulphate (natron-lakes), sometimes by their
proportion of borax (borax lakes). From a geological point of view
they may be divided into two classes--(1) those which owe their
saltness to the evaporation and concentration of water poured into
them by their feeders; and (2) those which were originally parts of
the ocean.
Salt and bitter lakes of terrestrial origin are abundantly scattered
over inland areas of drainage in the heart of continents, as in Utah
and adjacent territories of North America, and on the great plateau
of Central Asia. These sheets of water were doubtless fresh at first,
but they have progressively increased in salinity, because, though
the water is evaporated, there is no escape for its dissolved salts,
which consequently remain in the increasing concentrated liquid. In
Ladâkh, extensive lakes formed by the ponding back of valley waters
by alluvial fans have grown saline and bitter, and have become the
site of deposits of rock-salt and soda.
The Great Salt Lake of Utah, which has now been so carefully studied
by Gilbert and other geologists, may be taken as a typical example
of an inland basin, formed by unequal subterranean movement that
has intercepted the drainage of a large area, wherein rainfall
and evaporation on the whole balance each other, and where the
water becomes increasingly salt from evaporation, but is liable to
fluctuations in level, according to oscillations of meteorological
conditions. The present lake occupies an area of rather more than
2,000 square miles, its surface being at a height of 4,250 feet above
the sea. It is, however, merely the shrunk remnant of a once far more
extensive sheet of water, to which the name of Lake Bonneville has
been given by Gilbert. It is partly surrounded with mountains, along
the sides of which well-defined lines of terrace mark former levels
of the water. The highest of these terraces lies about 940 feet
above the present surface of the lake, so that when at its greatest
dimensions this vast sheet of water must have stood at a level of
about 5,200 feet above the sea, and covered an area of 300 miles from
north to south, and 180 miles in extreme width from east to west.
It was then certainly fresh, for, having an outlet to the north, it
drained into the Pacific Ocean, and in its stratified deposits an
abundant lacustrine molluscan fauna has been found. According to
Gilbert there are proofs that, previous to the great extension of
Lake Bonneville, there was a dry period, during which considerable
accumulations of subaerial detritus were formed along the slopes of
the mountains. A great meteorological change then took place, and the
whole vast basin, not only that termed Lake Bonneville, but a second
large basin, Lake Lahontan of King, lying to the west and hardly
inferior in area, was gradually filled with fresh water. Again,
another meteorological revolution supervened and the climate once
more became dry. The waters shrank back, and in so doing, when they
had sunk below the level of their outlet, began to grow increasingly
saline. The decrease of the water and the increase of salinity
were in direct relation to each other until the present degree of
concentration has been reached. The Great Salt Lake, at present
having an extreme depth of less than 50 feet, is still subject to
oscillations of level. When surveyed by the Stansbury Expedition
in 1849, its level was 11 feet lower than in 1877, when the Survey
of the 40th Parallel examined the ground. From 1866, however, a
slow subsidence of the lake has been in progress, consequent upon a
diminution of the rainfall. Large tracts of flat land, formerly under
water, are being laid bare. As the water recedes from them and they
are exposed to the remarkably dry atmosphere of these regions, they
soon become crusted with a white saliferous and alkaline deposition,
which likewise permeates the dried mud underneath. So strongly
saline are the waters of the lake, and so rapid the evaporation,
as I found on trial, that one floats in spite of one’s self, and
the under surfaces of the wooden steps leading into the water at
the bathing-places are hung with short stalactites of salt from the
evaporation of the drip of the emergent bathers.
Some of the smaller lakes in the great arid basin of North America
are intensely bitter, and contain large quantities of carbonate
and sulphate as well as chloride of sodium. The Big Soda Lake near
Ragtown in Nevada contains 129.015 grammes of salts in the litre of
water. These salts consist largely of chloride of sodium (55.42 per
cent of the whole), sulphate of soda (14.86 per cent), carbonate of
soda (12.96 per cent), and chloride of potassium (3.73 per cent).
Soda is obtained from this lake for commercial purposes.
Salt lakes of oceanic origin are comparatively few in number. In
their case, portions of the sea have been isolated by movements of
the earth’s crust, and these detached areas, exposed to evaporation,
which is only partially compensated by inflowing rivers, have shrunk
in level, and at the same time have sometimes grown much salter than
the parent ocean.
The Caspian Sea, 180,000 square miles in extent, and with a maximum
depth of from 2,000 to 3,000 feet, is a magnificent example. The
shells living in its waters are chiefly the same as those of the
Black Sea. Banks of them may be traced between the two seas, with
salt lakes, marshes, and other evidences to prove that the Caspian
was once joined to the Black Sea, and had thus communication with
the main ocean. In this case, also, there are proofs of considerable
changes of water-level. At present the surface of the Caspian is 85½
feet below that of the Black Sea. The Sea of Aral, also sensibly salt
to the taste, was once probably united with the Caspian, but now
rests at a level of 242.7 feet above that sheet of water. The steppes
of southeastern Russia are a vast depression with numerous salt lakes
and abundant saline and alkaline deposits. It has been supposed that
this depression continued far to the north, and that a great firth,
running up between Europe and Asia, stretched completely across what
are now the steppes and plains of the Tundras, till it merged into
the Arctic Sea. Seals of a species (_Phoca caspica_) which may be
only a variety of the common northern form (_Ph. fætida_) abound in
the Caspian, which is the scene of one of the chief seal-fisheries of
the world.[2] On the west side of the Ural chain, even at present, by
means of canals connecting the rivers Volga and Dwina, vessels can
pass from the Caspian into the White Sea.[3]