historic geology:! 127 



rain and frost, rivers, waves and currents through all time has 

 led to the deposition of a succession of strata which on the 

 whole are unbroken in their sequence, though they have varied 

 so much in the areas of their deposition that in no region do 

 we find the series complete. There has been a break of con- 

 tinuity in those areas which for a time were elevated above the 

 sea, but the continuity of the geologic series has always been 

 maintained in cue area or another. Contemporaneous strata 

 are found only in those areas which are simultaneously depressed 

 and which were submerged during the same time. 



Contemporaneous strata may differ widely in composition 

 owing to differences in material and the conditions of their de- 

 position. Thus the Potsdam sandstone in northern New York 

 is contemporaneous with a limestone in Saratoga and Dutchess 

 counties. 



As a result of the alternating invasions and retreats of the 

 ocean over the land, we find in various geologic systems what is 

 known as a trinity of formations, viz. a base consisting of sand- 

 stone or conglomerate, a center consisting chiefly of limestone and 

 a summit of shale or mud stone. 



The cause of this alternation is not fully known. The sand- 

 stones and conglomerates are usually solidified beach and shoal 

 water deposits. The shales are solidified sea bottom de- 

 posits consisting of the finer material carried from the shore 

 by waves and currents and also of sediment carried into the sea 

 by rivers. 



The limestones were probably formed in many cases as at the 

 present day, in warmer waters, which permitted the luxuriant 

 growth of corals, mollusks and other marine invertebrates which 

 have external skeletons composed of carbonate of lime. In New 

 York there were coral reefs in the Trenton, Niagara and Cornif- 

 erous periods. Whether corals in Palaeozoic time required the 

 same warm temperature of water as at the present day, we do 

 not know. ■ 



a Geikie's Text Book of Geology, Illrd Ed. p. 454. 



