650 ELIOT BLACKW ELDER 



north the comparative scarcity of PHocene strata renders the tracing 

 of the folding in that direction more difl&cult. In the region of 

 the Antilles, Pliocene strata are much more abundant, but there 

 they are apparently not folded and only very gently tilted. 



GENERAL CONSIDERATIONS 



The table of orogenic epochs given above is still far from 

 complete, and tabulations of this kind are necessarily subject to 

 correction. The most evident defects relate to the pre-Cambrian. 

 Until the pre-Cambrian terranes in widely separated districts have 

 been correlated much more securely than at present, we shall not 

 know how many orogenic epochs fell within the Archeozoic or even 

 the Proterozoic era; and we shall know but little of the true extent 

 of those ancient orogenies which are already recognized as facts. 

 Again, the effects of some orogenic movements, especially near the 

 borders of the continent, have been so masked by later crumplings 

 or have been so largely buried beneath younger sediments or by the 

 sea itself, that the record, once clear, is now largely destroyed. 

 Thus we know nothing of the southwestward extension of the 

 Appalachian system of folds beneath the sediments of the Gulf 

 coastal plain; and on the Pacific slope the complex structure and 

 metamorphism render it difficult to get information about any 

 orogenic epochs which that region may have suffered before the 

 Jurassic. Some of these limitations will gradually be removed as 

 the making of critical local studies progresses. 



In most of the known epochs of orogeny, a single elongate belt 

 was affected, while much broader surrounding regions experienced 

 nothing more revolutionary than changes of level, gentle warping, 

 a little faulting, and sometimes volcanic activity. Most of the 

 folded belts were thousands of miles in length and a few of the best- 

 known measured many thousands. The Laramide system of the 

 Americas extended over about half of the earth's circumference, 

 and the Miocene folded belts are long enough to reach more than 

 one and a third times around the globe. 



The belts vary from 50 to more than 600 miles in width and in 

 Asia locally exceed 1,000 miles. The average width of the crumpled 

 zones is, however, between 100 and 300 miles. Where the folding 



