iSSS-] 505 [Crosby. 



possibly not more than two or three inches in a million years. 

 Now suppose that after a submergence of ten million years the 

 floor of the deep ocean is slowly raised to form dryland. Is it sur- 

 prising that a bed five or possibly ten feet thick of ferruginous clay, 

 containing organic remains similar to those found in shore deposits, 

 is not recognized as of abyssal origin, but is completely lost among 

 the hundreds of feet of marginal sediments composing the new con- 

 tinent? In the ordinary sense, there are no abyssal sediments, 

 but we find over these oceanic wastes merely the impalpable dust 

 which slowly settles during the lapse of countless ages from the 

 limpid water of the central sea. The land is the great theatre of 

 erosion and the sea of deposition ; but just as there are extensive 

 rainless tracts on the continents where there is practically no ero- 

 sion, so there exist still larger areas of ocean-floor over which the 

 complementary process, or deposition, approaches the vanishing 

 point. On both land and sea the main field of geological opera- 

 tions is marginal, following the shore-line ; but nowhere does the 

 earth's crust experience such perfect rest as under the deep sea. 



The application of this principle to the explanation of the con- 

 formable contact of the Potsdam and Carboniferous over what must 

 have been the central and abyssal areas of the Paleozoic sea is 

 obvious. The first invasion of this sea spread over its floor a con- 

 tinuous sheet of Potsdam sandstone, which usually becomes cal- 

 careous upward. But, while the eastern and western marginal 

 areas of the sea-floor continued to be the theatres of rapid deposi- 

 tion, the central areas soon became abyssal in depth and in the 

 character of the deposits. That is, deposition was reduced to a 

 minimum, or virtually suspended, over considerable tracts, during 

 the Ordovician, Silurian and Devonian ages. When the movement 

 of the crust was finally reversed, and the sea bottom was elevated 

 during the close of Paleozoic time, the Carboniferous limestones 

 were formed, passing up, usually, as in the Black Hills, into argil- 

 laceous and arenaceous strata which indicate a very marked shal- 

 lowing of the sea. According to this interpretation, therefore, the 

 stratigraphic record is continuous, but the middle chapters are so 

 condensed as to be almost illegible, the history of Ordovician, Si- 

 lurian and Devonian times in the Black Hills being, apparently, 

 represented wholly by the few feet of ferruginous and highly argil- 

 laceous limestone separating the typical Potsdam sandstone below 

 from the pure Carboniferous limestone above. Prof. Franklin R. 



