646 ELIOT BLACKW ELDER 



and hence of doubtful value in correlation. The various writers on 

 Pacific coast geology are, therefore, by no means in harmony as to 

 the ages of some of the strata. The existence of a conspicuous 

 angular unconformity is conceded. Above this conformity lies 

 either the Chico (upper Cretaceous) or certain Eocene formations. 

 Below it lie much more highly folded and locally even metamor- 

 phosed rocks which may in general be referred to as the Aucella 

 beds. These have yielded both invertebrates and plant remains. 

 The former have been classified usually as lower Cretaceous, but 

 sometimes as late Jurassic; the plants nearly always as Jurassic. 

 At present the balance of qualified opinion seems to be in favor 

 of regarding the system as largely Comanchean but in part late 

 Jurassic. This orogeny, if it was a fact, is among the least-known 

 in the later part of the earth's history. 



Laramide orogeny {post-Cretaceous). — The deformative move- 

 ment which gave birth to the Rocky Mountain system is one of the 

 three best-known events of its kind in North America. The folds 

 thus produced trend north by west from southeastern Mexico to 

 western Texas, thence across New Mexico, Colorado, Wyoming, 

 Montana, western Alberta, and the northwest provinces of Canada 

 into northern Alaska, finally reaching the Arctic Ocean at Cape Lis- 

 burne. Although there may have been folding in Central America 

 also at this time, the fact that the same region suffered a further 

 crumpling in the midst of the Tertiary has largely if not entirely 

 obliterated evidences of earlier deformation there. The post- 

 Cretaceous folds reappear, however, in full force in the Andean 

 system of South America. On the eastern or inland side of the 

 system the limits are generally distinct, for the folds pass somewhat 

 abruptly into the nearly horizontal strata of the Great Plains. 

 There are, however, some outlying tracts of disturbance character- 

 ized by gently tilted strata and laccolithic intrusions, as in the Black 

 Hills of South Dakota and the Uvalde district of Texas. The same 

 general condition seems to prevail throughout the extent of the 

 system. On the westward or Pacific side the limits are much less 

 definite because the post-Cretaceous folds are not easily distinguished 

 from those of the Nevadian epoch, except where late Cretaceous 

 strata are present; and it is an embarrassing fact that outcrops of 



