380 PALEOZOIC TIME. 



In the Eastern-border region, about the Gulf of St. Lawrence 

 (which was probably an interior basin like the Interior Continental), 

 there were limestones forming almost continuously, from the Calcif- 

 erous epoch in the Lower Silurian to the close of the Clinton epoch 

 in the Upper Silurian, which is the last of the formations there 

 observed. With regard to other parts of the Eastern-border region, 

 our knowledge is yet imperfect, and in great measure because the 

 crystallization which the rocks have undergone has obliterated most 

 of their original features. This is the case over New England and 

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

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

 continental plateau, is still under water (p. 11). The map, Fig. 735, 

 gives a general view of the breadth and depth of this plateau, off the 

 coast of New Jersey. 



3. Diversities in the different regions as to the thickness of the rocks. 

 — The maximum thickness of the North American Paleozoic rocks 

 is 55,000 feet. About 45,000 feet of this thickness occur in the Ap- 

 palachian region of Pennsylvania, the rest being made up by the 

 excess of the Carboniferous formation in Nova Scotia. All this 45,000 

 feet 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. The general thickness over the Ap- 

 palachian regions is 40,000 feet, according to Hall. Each of the suc- 

 cessive 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 thick- 

 ness varies from 3,500 (and less on the north) to 6,000 feet. It is, 

 therefore, from one seventh to one twelfth that in the Appalachian 

 region. 



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

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

 the fragmental Huronian beds of the closing part of the Archaean age 

 accumulated to a thickness of 10,000 to 20,000 feet ; and, in the course 

 of the Canadian period, the sedimentary beds, in some places about 

 the former lake, reached a thickness of 3,000 to 4,000 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 beds in Canada are three times as thick, or nearly 1,000 

 feet. 



In Missouri, during the Calciferous and Quebec epochs, the accumu- 

 lations had the great thickness of 1,300 feet, — an exception to the 

 usual fact in the Interior Continental region. 



4. Relative duration of the Paleozoic ages. — The thicknesses of 



