CLASSIFICATION GRADATIONAL AND LITHOLOGIC CRITERIA 469 



assumed at the close of the Ordovician and another at the close of the 

 Tennessean. There has been no greater or rather more general break in 

 the sedimentary record of the continents and in the evolution of fossil 

 marine organisms than occurred in the time between the closing stages 

 of the Ordovician and middle stages of the Silurian. Through vig- 

 orous diastrophism probably was largely confined to areas close to the 

 margin of the continents, where its evidence has since be^n obliterated or 

 buried (see pages 435 to 440), there is yet positive testimony of folding 

 and erosion in eastern New York and New England that must have oc- 

 curred about this time. In the Mississippi Valley and thence westward 

 to the Great Basin slight elevation, beginning locally with the close of the 

 Black Eiver, but accomplishing complete emergence only at the close of 

 the Trenton, seems to have prevailed to the beginning of the Eich- 

 mondian. During this emergent phase baseleveling, as described on page 

 308, was in progress in the Mississipi Valley. In the Appalachian region 

 shale deposition began in many parts of the valley during the late Cha- 

 zyan, and before the close of the period heavy sandstone formations 

 (Oswego and Juniata), thought to be land deposits, were laid down in 

 the northwestern portion of the valley in Pennsylvania and New York. 

 Sandstone deposition, with possible change in climate, continued here 

 into the Silurian, which began in this region with the Tuscarora quart/ite 

 ("white Medina''). Thick deposits of shale and sandstone, the latter 

 more local in distribution than the former, were deposited about this time 

 also in the Ouachita geosyncline in western Arkansas and eastern 

 Oklahoma. 



The fact of prime significance brought out by study of the deposits in 

 the principal areas of Eopaleozoic rocks is that the predominatingly 

 clastic lower Cambrian deposits were followed by great series of lime- 

 stone strata containing, except locally, but a small percentage of frag- 

 mental matter. In the Appalachian Valley these limestones attained an 

 aggregate thickness of fully 15,000 feet before the late Ordovician and 

 early Silurian deposition of highly clastic material set in. The ex- 

 traordinary importance of this fact is apparent when we consider that 

 in the intervening relatively quiescent stages the valley was repeatedly 

 subjected to warping and consequent sea shifting, while on at least three 

 occasions complete sea withdrawal occurred. Obviously, the movements 

 which occasioned these intervening oscillations were inferior in vigor and 

 extent of areas prominently affected than was the early Cambrian de- 

 formation which preceded and the early Silurian which succeeded them. 

 That the latter effected unusual elevation even in the interior areas of 



