Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/293

Rh LAKEs.] § 4-. Lakes. 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 oftcn of large size, in warmer latitudes. They do not belong to the nor- mal system of erosion in which ru.nning water is the prime agent, and to which tl1e excavation of valleys and ravines nmst be attributed. On the contrary, they are exceptional to that system, and the constant tendency of running water is to fill them up. Their origin, therefore, must be sought among some of the other geological processes. See part vii. 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 11s11ally salt. 1. FI'€S7L-2£'((f€7' Lal-cs.—Tliese, in a vast number of cases, are simply depressions or expansions of the valleys in which they lie. They receive a river at the upper end, together probably with many minor t.rib11taries froln the sides, and let the accumulated waters overﬂow at the lower end. In all these cases, they act as filters for the river water, allowing its sediment to settle, and discharging it puriﬁed at the outﬂow, to continue its co11rse of erosion and mud- making down the valley. A river which ﬂows through a succession of lakes cannot carry much sediment to the sea, unless it has a long course to run after it has passed the lowest lake, a.nd receives one or more muddy tributaries. But the sediment which would have reached the sea and gone to form a delta or deposit on the sea—bed serves its purpose i11 contributing to ﬁll up the lakes. Hence it is rare to find any lake with an inﬂowing and outflowing river, where proofs of the gradual encroaclunent of the land upon the water may not be gathered. In other cases lakes do not lie in the natural drainage lines of a country, but are scattered apparently at random over the surface, fed by springs, rains, and streamlets from the slopes, or, if of large size, lying as great basins, receiving the collected waters of a wide region, and forming in this way the source out of which full—formed rivers emerge. From the little tarns of V-ales and the lochans of Scotland a series may be traced, through i111n1mer- able grades of size and form, in Scandinavia, Finland, and Canada, till we reach such vast sheets of inland water as Lakes Huron, Erie, and Superior, and the great equatorial lakes of Africa. I11 lakes of this kind also the process of ﬁlling 11p may often be traced. Each tributary stream pushes its delta into the water. 'here the lakes are shallow, and lie in temperate countries, vegetation comes in to aid in the conversion of the water, first into marsh, then into peat—bog, and ﬁnally into dry ground. In Scotland during the last few centuries great changes of this kind have been going on. 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 the other familiar features of the meeting line between land and sea reappear along the margins of such great fresh—water seas as Lake Superior. Three geological functions of lakes are (1) to arrest and equalize the drainage by regulating the outﬂow and pre- venting or lessening the destructive effects of ﬂoodsl ; to ﬁlter river water and permit of the undisturbed accumu- lation of new deposits, which in some modern cases may cover thousands of square miles of surface, and might attain 1 Vinds, by blowing strongly down the length of a lake, sometimes considerably increase for the time being the volume of the outflow. If this takes place coincidently with a heavy rainfall, the flood of the river is greatly augmented. These features are noticed in Loch Tay (D. Stevenson, Reclamation of Land, p. 14). Hence, though, on the whole, lakes tend to moderate ﬂoods in the outﬂowing rivers, they may by a combination of circumstances sometimes increase them. GEOLOGY 279 a thickness of nearly 3000 feet (Lake Superior has an area of 32,000 square miles ; Lago Maggiore is 2800 feet deep) ; (3) to furnish an abode for a lacustrine fauna and ﬂora, to receive the remains of the plants and animals washed down from the surrounding country, and to entomb all these or- ganisms in the growing deposits, so as to preserve a record of the terrestrial life of the period. The deposits in lakes consist of alternations of sand, silt, mud, and gravel, with occasional irregular seams of vegetable matter, and layers of calcareous marl formed from the accumulation of lacustrine shells, Entomostraca, &c. In a lake receiving much sedi- ment there will be little or no marl formed, at least not during the time when the sediment is being deposited. In clear lakes, on the other hand, where there is very little sedi- ment or where it only comes occasionally at wide intervals of ﬂood, beds of white marl, formed entirely of organic re- mains, may gather on the bottom to a depth of many yards. 2. Salt La/res may be divided into two cla.s.ses—(a) those which owe their saltness to the evaporation and con- centration of the fresh water poured into them by their feeders ; and (1)) those which were originally parts of the ocean. Salt lakes of the ﬁrst kind are abundantly scattered over the inland areas of drainage in the heart of co11ti- nents, as in the great Lake of Utah, and numerous other minor lakes in North America, and the abundant salt lakes of the great plateau of Central Asia. These sheets of water were doubtless fresh at ﬁrst, but they have progressively in- creased i11 'salinity, because though the water is evaporated as fast as it is received, there is no escape for the dissolved salts, which consequently remain in the increasingly concen- trated liquid. Salt lakes of the second class are compara- tively few in number. In their case portions of the sea have been isolated by movements of the earth’s crust, a.nd 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 2000 to 3000 feet, is a magniﬁcent example. The shells are chieﬂy the same as those still living in the Black Sea. Banks of them may be traced between the two seas, with salt lakes and marshes and other evidence to prove, not only that the Caspian was once joined to the main ocean, but that a great ﬁrth ran up between Europe and Asia, and possibly stretched completely across what are now the steppes and plains of the Tundras till it merged into the Arctic Sea. Even at present, by means of canals connect- ing the rivers Volga and Dwina, vessels can pass from the Caspian into the White Sea. But the surface of the Caspian is now more than 80 feet below that of the Black Sea. At present the amount of water supplied by rivers to the Caspian just balances that removed by evaporation, so that the level appears to be no longer sinking. But though, owing to the enormous volume of fresh water po11red into it by these rivers, the Caspian is 11ot as a whole so salt as the main ocean, and still less so than the Mediterranean, nevertheless the inevitable result of evaporation is there manifested. Along the shallow pools which border this sea a constant deposition of salt is taking place, forming sometimes a pan or layer of rose-colo11red crystals on the bottom, or gradually getting dry, and covered with drift sand. This con- centration of the water is still 111ore marked in the great offshoot called the Karaboghaz, which is connected with the middle basin of the Caspian by a channel 1 -50 yards wide and 5 feet deep. Through this narrow month there ﬂows from the main sea a constant current, which Von Baer estimated to carry daily into the Karaboghaz 350,000 tons of salt. An appreciable increase of the saltness of that gulf has been noticed: seals, which once frequented it, have forsaken its barren shores. Layers of salt are gathering on the mud at