NEGATIVE ELEMENTS OF NORTH AMERICA 399 



In the Appalachian trough opportunity for maximum sedimentation 

 during the Paleozoic was afforded by the profound subsidence of an area 

 in New York, Pennsylvania, Virginia, and Ohio. At Mauch Chunk the 

 totar thickness is approximately 30,000 feet. The, Devonian subsidence 

 alone was 10,000 feet, and the area of the sunken element is well indi- 

 cated by the mussel-shaped form which the Hamilton black shale had 

 assumed by the close of Devonian time.* This element presents a striking 

 contrast with Appalachia, the positive mass which bounds it on the east. 

 The maximum vertical displacement of the negative with reference to the 

 positive can scarcely have been less than 40,000 feet during the Paleozoic 

 era. Let us call this sunken area the northern Appalachian negative 

 element. 



A second area of notable subsidence occupies the southern end of the 

 Appalachian trough. It developed during the Ocoee epoch (late pre- 

 Cambrian or Cambrian) and continued throughout the Paleozoic to be a 

 region of more or less subsidence and deposition. The total thickness of 

 strata, including the Ocoee, is probably 20,000 feet, and the displacement 

 with reference to Appalachia was probably not less than 25,000 feet. 

 The area comprises part of western North Carolina, northern Georgia 

 and Alabama, and eastern Tennessee. We may call this negative element 

 the southern Appalachian. 



The northern and southern Appalachian negative elements were ap- 

 parently distinct, although the zone of Paleozoic sediments and Appa- 

 lachian folding connects them; but in southern Virginia there are 

 unconformities in the sedimentary series which give the district a dis- 

 tinct character with a less decided negative tendency. 



The Rocky Mountain trough, as Walcott called it, occupies the Great 

 Basin region in the United States and the ranges in British Columbia 

 between the Great plains and the Columljia river, together with their 

 extensions southward into Idaho. Between the Great basin of Nevada 

 and soutliern Idaho, there lies in central Idaho and western Montana a 

 district in which the great thicknesses of pre-Paleozoic and Paleozoic sedi- 

 ments appear to be wanting. The trough is thus apparently divided into 

 a southern and a northern part. 



Over the Great Basin element, as we may call it, Paleozoic sediments 

 accumulated to a thickness of 32,000 feet, according to King.f The 

 Eureka section comprises 30,000 feet. J In contrast with' these great 



* Bailey Willis : Paleozoic Appalachia. Maryland Geological Survey Reports, vol. 4, 

 pp. 61-62, pi. iv. 



t Clarence King : Geological Survey of the Fortieth Parallel, vol. 1, Systematic Geol- 

 ogy, p. 246. 



t Arnold Hague : Geology of the Eureka district. U. S. Geol. Survey, Monograph xx, 

 p. 208. 



