Time-table for North America. 5 



During the past twenty years much has been done in deciph- 

 ering the historical geology of North America, and as this 

 chronology needs to be adapted as far as possible to the older 

 European sequence, it is well to put the scattered information 

 into better order. The scheme here offered for North America 

 does not as a rule go into detail beyond the "systems" of rocks 

 or the "periods" of time, and it is also our desire to stand by 

 the old and well-known terminology as far as possible. 



We may well begin by repeating the questions asked by Suess 

 in 1885 : What constitutes a period and what determines its 

 beginning and its end ? How does it happen that these strati- 

 graphical subdivisions extend over the whole earth? His 

 answer then was: "If we could assemble in one brilliant tri- 

 bunal the most famous masters of our science and could lay 

 this question before them, I doubt whether the reply would be 

 unanimous, I do not even know if it would be definite." While 

 in this Twentieth Century we are coming much nearer to a 

 definite answer, and see more clearly the several principles 

 which serve as a basis for determining the earth's history, still 

 a fixed geological chronology is not yet established. 



The evidence of fossils. — Fossils furnish the first step in the 

 process of stratigraphic correlation. Their testimony is checked 

 by the geographic distribution of the sediments that contain 

 them and the relation of the latter to the formations beneath 

 and above them (superposition). These principles are easy 

 to state but very difficult to apply accurately to so great a land 

 mass as North America, and even though approximately a cen- 

 tury of work has been devoted to it, the ground is only about 

 half covered by detailed studies. 



In general, sedimentation is a slow process, and by the time 

 one foot of average rock accumulates, probably a thousand 

 generations of marine invertebrates have appeared, passed 

 their life on to their descendants, and vanished. Under 

 relatively constant surroundings, it is held that but little if any 

 recognizable change in the species is developed, but as the 

 environment of the organisms is continually changing, even 

 though only to a minor extent, these physical alterations cause 

 the faunas (animal associations) at the very least to alter their 

 combinations and to shift from place to place. They die out 

 in one area, but gain a foothold elsewhere, and although this 

 to-and-fro migration is slow when measured in years, yet in 

 stratigraphy the faunal assemblages appear as if suddenly 

 introduced. This fact has always excited the interest of the 

 paleontologist, and he has explained the phenomena according 

 to the view of his generation. Once he thought them due to 

 special creations of new types or recoinages of old, but since 

 the time of Darwin they have been looked upon as slow evolu- 



