GENERAL OBSERVATIONS. 385 



border of the continent south of New York. Besides this, a strip 

 01 land some eighty miles wide, constituting the eastern margin of 

 the continental plateau, is still under water (p. 12). The map, 

 fig. 664, gives a general view of the breadth and depth of this pla- 

 teau off the coast of New Jersey. 



4. Diversities in the different regions as to the thickness of the rocks. — The 

 maximum thickness mentioned on p. 377 — 50,000 feet — occurs in 

 the Appalachian region. It is not found in any one place ; for 

 some of the formations are thickest along the middle of the region, 

 others on the western side, and still others on the eastern ; and, 

 again, the beds of the Carboniferous age are most largely developed 

 to the northeast in Nova Scotia. Owing to facts like these, the 

 maximum amount exceeds considerably the actual thickness of the 

 accumulations over the region. Still, it cannot be less than 36,000 

 feet, or 6£ to 7 miles. Each of the successive formations in the 

 Appalachian region is remarkable for its great thickness from the 

 Potsdam upward. 



In the central portions of the Interior Continental basin the 

 thickness varies from 3500 (and less on the north) to 6000 feet. 

 It is, therefore, from one-sixth to one-tenth that in the Appalachian 

 region. 



Another region of unusual thickness lies on the north side of the 

 Interior basin, near the Azoic. Along Lakes Superior and Huron 

 the fragmental Huronian beds in the closing part of the Azoic age 

 accumulated to a thickness of 10,000 feet ; and in the latter part of 

 the Potsdam period the Calciferous beds in some places about the 

 former lake have a thickness of 3000 to 4000 feet. Again, in the 

 region of the St. Lawrence, about Ottawa, the Potsdam beds have 

 twice the thickness they exhibit in the State of New York, and the 

 Trenton in Canada are three times as thick, or nearly 1000 feet, — an 

 unusual thickness for a limestone formation. 



In Missouri, during the Calciferous epoch, in the Potsdam period, 

 the accumulations had the great thickness of 1300 feet, — an excep- 

 tion to the usual fact in the Interior Continental region. 



Relative duration of the Paleozoic ages. — The thickness of the series 

 of rocks pertaining to the several ages affords some data for esti- 

 mating their time-ratios. The results are necessarily uncertain, since 

 the increase of a rock is often directly connected with the subsi- 

 dence there in progress, as has been elsewhere explained. Still, 

 the conclusions are sufficiently reliable to be here presented. 



Taking the maximum thickness along the Appalachians of the 

 successive formations (the limestone and fragmental beds in each 

 case from the same region), we find for the 



