
394 DYNAMICAL GEOLOGY. — [Boox TM. 
energy of the waves by which it is produced. This subaqueous 
platform is well developed in the Lake of Geneva. 
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 (p. 174)." 
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 terrestrial life of the period 
during which they continue. It is as receptacles of sediment and 
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, and 
eravel, with occasional irregular seams of vegetable matter, and 
layers of calcareous marl formed from the accumulation of lacustrine 
shells, Hntomostraca, &c. 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, beds of white marl, formed 
entirely of organic remains, may gather on the bottom to a depth 
of many yards, 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 
Kocene 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 dis- 
appearance 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 (p. 373), but where it is performed by the retrocession 
of a waterfall at the head of an advancing gorge it may be relatively 
rapid.?_ It is usual to find in a river course 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. It is likewise common to meet with successive terraces 
fringing a lake and marking former levels of its waters. When we 
reflect upon the continued operation of the agencies which tend to 
_ efface them, the lakes now extant are seen to be necessarily of com- 
- crahhett recent date. Their modes of origin are discussed in 
ook VII. 
* For an elaborate paper on these lake-ores (See-erze) see Stapff, Z. Deutsch. Geol. 
Ges, xviii. pp. 86-173; also postea Section III. p. 462. 
? The level of the Lake of Geneva is said to have been lowered about six and a half 
fect since Roman times (Bull. Soc. Géol. France (3), iii. p. 140); but this may be explicable 
by diminution in the water supply. 
