11 



fresh water silts occur upon every continent, and are well 

 defined; but the very purity of these fresh waters forbids 

 large deposits, and they are generally determined by the 

 peculiar 23lacidity of their formations and the presence of 

 fresh water shells. 



Europe, which is the only continent that has no inland 

 salt sea cut off from oceanic circulation, has, however, a 

 wonderful witness to decreasing precipitation hi the glaciers 

 of the Alps, which are said to be of less extent than for- 

 merly; the ancient glaciers' lines of erosion and deposit 

 can be distinctly traced, notwithstanding the obliterating 

 eiiects of atmospheric agencies. 



So remarkable are the evidences of glacial action in the 

 North temperate zone, l)oth in Europe and America, at a 

 period succeeding that of tropical warmth in the same 

 latitudes, that the glacial epoch is recognized as a distinct 

 geological era, and the causes which produced this sudden 

 change of temperature and other physical conditions have 

 never been satisfactorily explained. 



, It is probable that evaporation may account for it. If 

 there were barriers of land or of ice that cut off the 

 Arctic waters from free circulation with those of the tem- 

 perate zone — and the geological formations of the British 

 Islands indicate the removal of a large body of land there — 

 the tropical region being one of excessive' evaporation 

 and the North temperate and Arctic that of precipitation 

 and congelation, there woukl have been a rising of the 

 level of Arctic ices and waters and a lowering of level of 

 the waters of the South temperate and torrid zones, and 

 perhaps the emergence of land there. The removal of 

 such barriers either by igneous or aqueous forces, or by 

 the decrease of precipitation and congelation lowering ice 

 barriers, and the irruption of Northern waters, icebergs 

 and ice floes to re-establish the^ lost level of the waters 

 would account for the glacial deposits. The land re-emerg- 

 ing when evaporation once again had given the waters to 

 the advancing organic creation. 



