620 E. O. TTLRTCH T^EVTSTON OF THE PALEOZOIC SYSTEMS 



Great Basin sequence, but, assuming the competence of the diastrophic 

 evidence relied on, the answer in the Appalachian region must be in the 

 negative. Briefly reviewing the diastrophic history of the Appalachian 

 Valley during the Cambrian, we note first that most of the lower Cam- 

 brian areas were confined to narrow troughs which now form much of 

 the eastern or southeastern highland border of the Appalachian Valley 

 tract. At times these or intervening seas occupied large parts of the 

 valley itself. Although the deposits of this epoch occur at intervals in 

 the folded belt from Alabama to Newfoundland, it seems highly im- 

 probable that a continuous seaway extending throughout the length of 

 this belt existed at any time in geologic history. In my opinion, it seems 

 more in accord with known facts to regard the Appalachian trough as 

 having been broken up — during the lower Cambrian as in later periods — 

 into subordinate basins, each with one or more independent connections 

 with the Atlantic basins to the east. It is thought unlikely, further, that 

 all of these subbasins were submerged at the same time. Under this con- 

 ception, the Newfoundland and New Brunswick part or parts of the 

 trough were occupied during the lower Cambrian by waters distinct in 

 their oceanic connections and in part or wholly different in age from 

 the waters which at other times during the epoch laid down the lower 

 Cambrian quartzose sandstones, shales, and limestones in Vermont and 

 eastern New York, and the similar deposits which extend with occasional 

 interruptions from New Jersey to Alabama. The last long stretch, doubt- 

 less, was at times divided into two or more structurally distinct parts, the 

 Maryland and Tennessee l)asins (see pages 562 to 560), at least having- 

 been, even in that early period, in existence. Periodic surface oscillation 

 occasioned alternate submergence and emergence of one and then another 

 of these subbasins, deposition in each, therefore, being interrupted at 

 times varying from place to place and being resumed at similarly varying 

 times. 



The close of the lower Cambrian and the beginning of the middle 

 Cambrian is set at a time when east-west crustal shortening caused gen- 

 eral and probably long continued emergence and, following this, resub- 

 mergence with shifting of the areas subjected to sea invasion. Except 

 locally in the southern part of the Appalachian Valley tract the new seas 

 spread farther westward but failed, perhaps in a corresponding degree, 

 to extend as far eastward as the preceding lower Cambrian waters. They 

 differed, also very notably in that the area between the Harrisburg 

 axis in Pennsylvania and Saint Lawrence Eiver remained emerged, while 

 to the southward from the named axis on to Alabama the submergence 

 of the new trough seems to have been geographically continuous and 



