356 E. O. ULRICII REVISION OF THE PALEOZOIC SYSTEMS 



northward migration of southern Atlantic faunas appears reasonably 

 assured. 



Significance of black shale deposition — Distribution and characteristics 

 of the shale. — Typical, non-calcareous black shale deposits are found ap- 

 parently in the marine formations of all continents. They are not con- 

 fined to any particular era, but seem to have been laid down in one area 

 or another in most, if not all, of tlie geological periods. Partially review- 

 ing only the North American Paleozoic occurrences, we may begin with 

 the Lower Cambrian black slates in Vermont (the Georgia), in the Appa- 

 lachian Valley, and in the Cordilleran trough in western ISTevada. Then 

 there are good deposits of Canadian age in the Saint Lawrence Valley, in 

 Vermont and eastern New York, in west central Arkansas and in the far 

 west, and the same regions contain other thick beds ranging from early to 

 late Ordovician in age. Silurian shales of this kind are found in the 

 Ouachita area of Arkansas and in Alaska. The Utica is the first and the 

 Maquoketa the second to transgress far inland beyond the submarginal 

 troughs to which the earlier black shales are almost confined. The Devo- 

 nian includes some of the best known examples, notably the Genesee. The 

 Chattanooga, which is early Waverlyan in age, spreads more widely in the 

 interior basins than any other, being recognizable and of extraordinary 

 thickness in Oklahoma on the southwest and along Lake Erie, where it 

 is represented by the Cleveland and probably the Sunbury, in the north. 

 The Tennessean, so far as known, contains black shales only in northern 

 Arkansas. The Pennsylvanian, however, includes many relatively thin 

 beds besides a few that, like the Caney in Oklahoma and its extensions to 

 the south in Texas ("Bend shale") and to the north in Arkansas and 

 Missouri ( *^^coal-bearing shale" of the Morrow), are thick and widely 

 recognizable. 



Many of these black shales attain great thicknesses, especially in the 

 submarginal troughs, where apparently uninterrupted measures of 1,000 

 feet or more are not uncommon. A notable feature is the great areal ex- 

 tent of these formations, even when but 50 feet or less in thickness. Be- 

 fore beginning the discussion it should be said that calcareous black 

 shales, like the Marcellus and Hamilton formations, are not included. 



Objections to prevailing interpretations. — Black shale deposition lias 

 been variously interpreted. Some of the more notable instances have been 

 rather generally thought to indicate Sargasso-like conditions in a broad 

 open sea. Other writers, on the contrary, have recently come to the con- 

 clusion that they represent deposits in an "inclosed marine body . . . 

 of great depth and imperfect vertical circulation" (Clarke"^), or that they 



