REVISION OF STRATIGRAPHIC CLASSIFICATION 303 



and accuracy. The discussion of these conditions, which are highly im- 

 portant and bear directly upon problems of correlation, will be found 

 in Part II of this work. For present purposes it will suffice to say that 

 differential crustal movements and the consequent birth or rearrangement 

 of land and water areas are the prime causes of the apparent discord- 

 ances between contemporaneous faunas of neighboring and more widely 

 separated areas. In many cases also the abrupt local extinction of faunas 

 and their replacement by others of widely different origin is attributable 

 to the same cause. 



Uniform composition of luidely transgressing faunas. — Eeference has 

 already been made to several instances illustrating the essential constancy 

 of widely distributed faunas. Such constancy is usually confined to de- 

 posits within areas bounded by comparatively permanent lines of eleva-. 

 tion. In some instances, however, erosion and concomitant sea filling 

 produced a condition so closely approximating uniformity of surface 

 relief that submergence immediately succeeding spread far and wide 

 over the usual barriers. The Eichmond fauna is one of the greatest 

 and perhaps the most convincing of these widely dispersed faunas. While 

 this Richmond transgression affected areas chiefly to the west and north 

 of Mississippi Eiver, the Onondaga Devonian fauna may be cited as simi- 

 larly transgressing the area to the east of that river. The Onondaga is 

 a fine example of a thin limestone formation overlapping a nearly base- 

 leveled area. It failed to top either the Cincinnati or the Nashville 

 dome, and it seems to be absent, at least as a limestone, over a large part 

 of the Appalachian Valley; but, nevertheless, it is to be regarded as the 

 first great inland advance of the Devonian sea. 



The late Devonian, or, as I prefer to call it, the early Waverlyan, black 

 shale transgression represented, according to my interpretation, by the 

 lower Woodford of Oklahoma, the Chattanooga shale of Arkansas, Mis- 

 souri, and Tennessee, and the upper beds of the Ohio shale in Kentucky, 

 Indiana, and Ohio, is another good example. A fourth, hitherto not 

 properly appreciated, transgression, following a period of evident base- 

 leveling, occurred toward the close of the lower Pottsville. Elevation 

 and subsequent erosion of the southeastern part of the continent, prob- 

 ably accompanied by continental creep and accentuation of old submar- 

 ginal downwarps, resulted in enormous, somewhat local deposits of clastic 

 material in West Virginia and Alabama and in the area south of Arkan- 

 sas Eiver in Arkansas and Oklahoma. Later crustal movements intro- 

 duced a more general subsidence permitting the Pottsville sea to spread 

 rapidly northward to Ohio on the east and to Kansas and Illinois in the 



