CLASSIFICATION GRADATIONAL AND LTTHO1.OGIC CRITERIA 471 



Beginning with the Silurian, we see in the finely divided clastic matter 

 of the Richmondian deposits in the Mississippi Valley the rather scant 

 sedimentary result of erosion on the slightly elevated median lands. 

 The Niagaran deposits which follow them consist almost entirely of 

 limestone. In the Appalachian region, however, the lands were higher 

 and contributed clastic material to the valley troughs longer, so that 

 here we find sandstone deposition continuing locally to the close of the 

 Clinton. 



The succeeding Silurian and early Devonian formations in the Appa- 

 lachian Valley consist mainly of limestone, but, beginning with the 

 Oriskany and ending with the Waverlyan, the deposits in the middle 

 and northern parts of the valley consist almost entirely of shales and 

 sandstones. In the southern part, however, especially in Alabama, the 

 upper part of this interval is decidedly calcareous, sandstone and 

 shale being confined to the lower and middle portions. Tn the median 

 and far western parts of the continent the Devonian is represented 

 almost entirely by limestones. This is true also for the Waverlyan in 

 and to the west of the Mississippi Valley, but east of Illinois this system 

 comprises little else than shale and sandstone. From these facts it has 

 been inferred that considerable elevation of the northern and middle 

 parts of the Appalachian region occurred during the Devonian. Judging 

 from the great volume of deposits in Pennsylvania and southern New 

 York, the land contributing to this area doubtless attained greater alti- 

 tudes than did its southern extension. Indeed, there is reason to believe 

 that Appalachia was cut in two by submergence in the region of Chesa- 

 peake Bay. Further, since the larger part of these deposits is of late 

 Devonian age, we naturally conclude that the elevation of the land 

 reached its maximum during this time. That this maximum was in 

 anywise comparable to that of the Sierra Nevada of California, as 

 suggested by Willis,^^ seems improbable. On the contrary, it is thought 

 that this land never attained such altitudes and that the great mass of 

 sediment removed from it is owing rather to repeated elevation during 

 the Devonian and Waverlyan than to a single great upward movement. 



The Devonian deformative movements, as indicated by the character 

 and volume of clastic deposits of this age, were almost world-wide in 

 extent. Comparing these deposits in the several continents, the geolog- 

 ical history of this period seems to have been even less pacific in Europe 

 than in America and elsewhere. In western Europe especially volcanic 

 rock is frequently interbedded with the stratified deposits; and in the 



<^ Bailey Willis : Paleozoic Appalachia. Maryland Geological Survey, vol. 4, 1902, p. 62. 



