4 Schuchert and Bar veil — Revised Geologic 



father of oceans, in which the land was victorious, but only 

 after building and rebuilding marginal bulwarks of chain upon 

 chain of mountains, marked by volcanoes from Alaska to 

 Mexico City. This western region now embraces the grand 

 Cordilleras of the continent and also all of Mexico. 



Hence we see that North America has the form of an elon- 

 gate basin, widely open at the north and nearly closed at the 

 southeast ; though in the latter place there stood for a long time 

 a transverse mountain range, the Ouachitas, of which only the 

 bases now remain, and these in part concealed by later mantles 

 of sediments. The geology of the Arctic shelf sea is not yet 

 well known, but the sediments in the folded United States 

 mountains are of considerable thickness and extent. Between 

 these elevated margins lies the great basin, much of which has 

 always lain near sea-level, and over it, entering through the 

 gaps between the marginal uplands, the oceanic waters have 

 again and again flowed widely, to form interior shallow 

 seas. These floods have come from the four quarters, most 

 widely from the Pacific and Arctic, least from the Atlantic, 

 and most persistently from the Mexican mediterranean. The 

 partial deciphering of these multitudinous events in their orderly 

 sequence, in addition to the similar unravellings in Eurasia by 

 the geologists of that continent, has given the very imperfect 

 geologic time-table here presented. 



The basis of chronology. — The fundamental principle under- 

 lying all geologic endeavor is evolution, the oscillating but pro- 

 gressive changes wrought in the long ages, changes whose 

 interpretation leads to the history of the earth — the science of 

 Historical Geology. 



The earth develops as a whole, but the record is far from 

 being everywhere alike; even if it were so, it would not be 

 wholly accessible for study, because sheet upon sheet of rock 

 hides others below, and the atmospheric agencies have destroyed 

 much through erosion. Likewise, the more complete strati- 

 graphic record buried under the oceans is hopelessly lost. There- 

 fore the completed geologic record will eventually be put 

 together from the evidence of all places which are at present 

 land, mainly, however, from the northern hemisphere, because 

 this is preeminently the land hemisphere. Such history is 

 largely brought about through the periodic adjustments of 

 continental and oceanic areas settling down upon a shrinking 

 nucleus, and in so doing crushing the crust into great folds 

 which tend to rise, especially upon the margins of the continents. 

 Broad movements of a vertical nature also take place at times, 

 whereby the continents, being underlain by lighter rock mat- 

 ter, tend to warp up and restore the elevations destroyed by 

 erosion, while the sea floors, loaded with sediment, tend to sink. 



