Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Fig. 330.—Distribution of various sorts of deep-sea deposits. (Murray. Challenger Reports.)

Another substance which is somewhat widespread in the ocean-bottom is phosphate of lime, which occurs in various sorts of oozes, in the manganiferous nodules, in glauconite, and in independent nodules. Like the grains of glauconite, the grains of phosphate of lime appear to have started as concretions in shells, and to be the result of the reaction of organic matter on the contents of sea-water. The immediate source of the lime phosphate in the water appears to have been the shells or bones of the numerous animals living in the sea.

Secondary minerals made from the constituents of volcanic matter which has been decomposed occur not uncommonly in the bottom of the sea. These minerals belong to the general class of zeolites, phillipsite being the most abundant. Their distribution is somewhat wide, but their quantity is slight.

Unfortunately, knowledge of the deep-sea deposits is limited to their superficial layers. Soundings do not usually penetrate more than a few inches, or at most a foot or two.

Unlike shallow-water deposits, those of the really deep sea seem to find no correlatives in the known rock formations of the land.

LAKES.

Most of the phenomena of the ocean are repeated on a smaller scale in lakes. The waves of lakes and their attendant undertows and littoral currents are governed by the same laws and do the same sort of work as the corresponding movements of the ocean. Tides are absent, or insignificant, but slight changes of level, known as seiches,[184] have been observed in many lakes. They are probably caused by sudden changes in atmospheric pressure. While they are generally very slight, they frequently amount to as much as a foot, and occasionally to several feet. The seiches are oscillatory movements, and their period is influenced by the length and depth of the lake. They have been studied most carefully in Switzerland. Currents corresponding to those of the ocean are slight or wanting in lakes, but since most lakes have inlets and outlets, their waters are in constant movement toward the latter. In most cases this movement is too slow to be readily noted, or to do effective work either in corrasion or transportation. The work of the ice, on the other hand, is relatively more important in lakes than in the sea.

Changes taking place in lakes.—The processes in operation in lakes are easily observed and readily understood. (1) The waves wear the shores, and the material thus derived is transported, assorted, and deposited as in the sea, and all the topographic forms resulting from erosion or deposition along the seacoast are reproduced on their appropriate scale in lakes. (2) Streams bear their burden of gravel, sand, and mud into lakes and leave it there. (3) The winds blow dust and sand into the lakes, and in some places pile the sand up into dunes along the shores. (4) Animals of various sorts live in the lakes, and their shells and bones give rise to deposits comparable to the animal deposits in the sea. (5) Abundant plants grow in the shallow water about the borders of many ponds and lakes, and as they die, their substance accumulates on the bottom. (6) At the outlet the water is constantly lowering its channel. The lowering of the outlet is often slow, especially if the rock be coherent, for the outflowing water is usually clear, and therefore inefficient in corrasive work. These six processes are essentially universal, and all conspire against the perpetuity of the lakes. (7) In lakes where the temperature is low enough for ice to be formed, it crowds on the shores and develops phenomena peculiar to itself. The ice of the sea may work in similar ways, but its work is restricted to high latitudes. (8) In lakes in arid regions, deposits are often made by precipitation from solution. The first five and the last of these processes are filling the basins of the lakes. As the sediment is deposited, a corresponding volume of water is displaced, and, if there be outlets, forced out of the basins; the sixth process is equally antagonistic to the lakes, while the seventh has little influence on their permanence. Given time enough, these processes must bring the history of any lake to an end. The lowering of the outlet will alone accomplish this result if the bottom of the basin is above base-level. Many lakes have already become extinct, either through the filling or draining of their basins, or through both combined. The antagonism of rivers and lakes long ago led to the epigram “Rivers are the mortal enemies of lakes.” True as this statement is, it does not follow that lakes will ever cease to exist, for the causes which produce new lakes may be in operation contemporaneously with those which bring lakes now in existence to an end.

Lacustrine deposits.—The beds of sediment deposited in lakes are similar in kind, in structure, and in disposition to beds of sediment laid down in the sea, but river-borne sediment is more commonly concentrated into deltas, since waves and shore-currents are less effective. Even the limestone of the sea has its correlative in some lakes. Some of it was made of the shells of fresh-water animals which throve where the inwash of terrigenous sediment was slight, some of it from the calcareous secretions of plants,[185] and some of it was precipitated from solution.[186] Salt and iron-ore[187] deposits are also sometimes made in lakes.

Extinct lakes.—The former existence of lakes where none now exist may be known in various ways. If the lake basin was filled, its former area is a flat, the beds of which bear evidence, in their composition, their structure, and often in their fossil contents, of their origin in standing water. Such a flat is commonly so situated topographically that the basin would be reproduced if the lacustrine deposits were removed. To this general rule there might be exceptions, as where a glacier formed one side of the basin when it was filled. If the lake was destroyed by the reduction of its outlet, or by the removal of some other barrier, such as glacier ice, or by desiccation, shore phenomena, such as beaches, spits, etc., may be found. In time such evidences are destroyed by subaërial erosion, so that they are most distinct soon after the lake becomes extinct.