5 2 ° 



HISTORICAL GEOLOGY 



forced into the strata at this time. At this time, too, a large part 

 of the continent of North America was affected by movements of 

 greater or less strength, so that at the close of the era, dry land 

 extended from the Sierra Nevadas on the west to the " Fall Line" 

 on the east. 



With the formation of the Rocky Mountains, the fourth great range 

 of the North American continent came into existence ; the Taconics 

 being formed at the close of the Ordovician, the Appalachians at 

 the close of the Paleozoic, and the Sierra Nevadas at the close of the 

 Jurassic. 



Duration of the Mesozoic. — Many facts point to a great duration 

 for this era. (i) The erosion of an immense thickness of rocks from 

 the Appalachian Mountains and the reduction of the continent to a 

 peneplain was accomplished during the era and must have taken an 

 almost inconceivable length of time. (2) The first Sierra Nevada 

 Mountains, although formed in the latter half of the era, were not 

 only raised — possibly to a great height — but were also, later, 

 worn down to a peneplain. (3) During the Upper Cretaceous 

 alone, 24,000 feet of sediments — almost five miles — were deposited, 

 being worn from the land and carried little by little to the seas by 

 the streams. (4) The evolution in the animal and plant life of the 

 era is very striking, from the standpoint of both form and structure. 

 It does not seem possible that, under the conditions existing at that 

 time as we understand them, these changes could have been brought 

 about rapidly. 



No matter upon what basis the estimate is made; whether the 

 time necessary for the erosion of thousands of feet of strata, or that 

 required for the deposition of great piles of sediment, the length 

 of the era must have been enormous. An estimate of 9,000,000 

 years has been suggested, but should be taken merely as an approxi- 

 mation. It may be too large, or several millions of years too short. 



REFERENCES FOR PALEOGEOGRAPHY 



Chamberlin and Salisbury, — Geology, Vol. 3, pp. 1-38; 59-79; 106-130; 137-172. 

 Schuchkrt, Chas., — Paleogeography of North America: Bull. Geol. Soc. America* 



Vol. 20, 1 9 10, pp. 427-606. 

 Schuchert, Chas., — Climates of Geologic Time: Carnegie Institution of Washington, 



Pub. 192, 1914, pp. 280-284. 

 S( on, W. B., — An Introduction to Geology, pp. 657-667; 678-683 ; 700-713. 

 STANTON, T. \\\, Outlines of Geologic History (Willis and Salisbury), pp. 182-199. 



