452 STUDIES FOR STUDENTS 



hypothesis of the accumulation of such materials at the bottoms, or 

 within the margins, of shallow seas. 



Again, in the Lower Cambrian, according to Walcott, the eastern 

 side of the narrow Appalachian trough appears to have been a bold 

 and precipitous mountainous area. In the trough itself beneath the 

 Olcnellus sandstone occurs a great series of variegated shales.' These 

 are of course no proof of land accumulation, but the geographic rela- 

 tions of land and water were such as to suggest the possibility that 

 delta deposits may have been built out into a trough which later, 

 by a slackening of erosion or a greater subsidence, allowed the region 

 to pass from largely subaerial to marine conditions, the marine 

 transgression being marked by the deposit of the Olenellus quartz- 

 ites. 



In conclusion it is suggested that the Pre-Cambrian and Lower 

 Cambrian ages of wide continental extension offered conditions favor- 

 able for the accumulation of the several types of subaerial deposits, 

 and that in the interpretation of the mechanical sediments of those 

 times this possibility should be always held in mind. The presumed 

 absence of a fossilizable land fauna and flora in the life of those 

 periods would remove the possibility of proving a continental origin 

 through such secure means. 



Paleozoic epicontinental basins. — During the Eopaleozoic the con- 

 tinents became largely submerged, but in the Neopaleozoic partial 

 emergence was the rule, varying from fairly extensive land condi- 

 tions at times (in late Silurian and Lower Devonian), as has been 

 shown by Ulrich and Schuchert,^ to submergence possibly nearly 

 as complete as that of the Ordovician. At times mountain-making 

 forces and regional uplift operated on an extensive scale, as is wit- 

 nessed, for example, by the enormous mass of Upper Devonian sedi- 

 ments along the northeastern portion of the Appalachian trough 

 from which Willis computes that the uplift supplying these sediments 

 corresponded in volume to a mountain range similar to the Sierra 

 Nevada, 3 and the sediments deposited in the Old Red Sandstone 



1 "The North American Continent during Cambrian Time," Twelfth Annual 

 Report, U. S. Geological Survey, Part I (1891), pp. 536, 551. 



2 "Paleozoic Seas and Barriers in Eastern North America." TV". Y. State Mus., 

 Bull. No. j2, pp. 633-663, 1902. 



3 Paleozoic Appalachia, Maryland Geological Survey, Vol. IV (1902), p. 62. 



