LACUSTRINE STRATA. 137 



old volcanoes, or occupy central depressions in the area which has 

 been the core of a volcano. In the department of the Puy-de-D6me 

 there are 276 small lakes, formed more or less directly as a consequence 

 of. the accumulation of volcanic ashes or lava streams damming up 

 valleys, or due to changes of level ; but there are eighteen lakes still 

 existing or filled up with sediment which occupy the craters of extinct 

 volcanoes. In Central Italy several of these lakes of large size occupy 

 the sites of old volcanoes. North of Rome one is more than 6 miles, 

 and another 10 miles long. Others are seen in the Eifel. 



Thus lakes are to be attributed chiefly to the action of lateral com- 

 pression, which at once throws the earth's crust into folds, and heaves 

 it out of the sea. They rise with tablelands, like the Great Salt Lake, 

 which is 4000 feet above the sea ; or Lake Titicaca, which occupies 

 a depression in the tableland of Potosi, at a height of 13,000 feet 

 in the Andes ; and other lakes in Thibet rise even higher. When 

 a lake is depressed beneath the sea-level, its position is always a con- 

 sequence of evaporation ; the depression is only some 80 feet in the 

 case of the Caspian, but about 1300 feet in the case of the Dead 

 Sea. The tendency of rain, and rivers, and glaciers is rather to carry 

 material from a higher to a lower level than to excavate hollows, so 

 that no large number of lakes of magnitude is likely to be attributable 

 directly to erosive forces. 



Lacustrine Strata. Several of the British geological formations 

 are inferred to have been deposited in lakes. The oldest of such 

 formations is the Old Red Sandstone ; for throughout the region over 

 which it is spread there are no marine fossils, but plenty of land 

 plants with the remains of fish, some of them allied to types which live 

 at the present day in rivers and lakes ; and at Kiltorcan, in the south 

 of Ireland, a fossil shell has been found and referred to the freshwater 

 genus Anodon. Hence the Old Red Sandstone is inferred to have 

 been formed in great synclinal folds where the waters of the adjacent 

 continent drained down and accumulated. In like manner the Coal 

 Measures often speak incontestably to lacustrine conditions ; and the 

 fact that coal-beds succeed each other in vertical order, like the 

 evidences of old land-surfaces in a delta, strongly enforces the infer- 

 ence that the intervening beds of shale and current-bedded sandstone, 

 free as they usually are from typical marine fossils, were accumulated 

 in waters which may sometimes have been brackish, but which were 

 essentially lacustrine. 



Passing onward in time to the Trias, with its beds of rock salt 

 and gypsum, we have another formation which gives evidence of 

 conditions which certainly included the evaporation^ and drying up 

 of salt lakes; for the footprints, rainprints, and ripple-marks in the 

 Cheshire Sandstones are more likely to belong to an ancient lake 

 margin than to the sea. The freshwater and estuarine beds of the 

 Lower Oolites of Yorkshire and Lincolnshire are quite as likely to 

 have been lacustrine, though the lake certainly was not elevated on a 

 tableland, as may have been the case with some of the salt lakes of the 

 Trias. In the Purbeck formation we have evidence of lakes in which 



