ORIGIN OF THE GENESEE SHALE 953 



taking place. From the thick character of the muds in eastern Pennsyl- 

 vania it would appear that this part of the State was comprised within 

 the estuary in which these black muds were laid down. That the waters 

 of this estuary were more fresh than salt is indicated by the scarcity of 

 marine organisms and by the dwarfed character of those that are found. 

 The source of the mud must have been in a region of low relief, where 

 residual soils rich in decaying vegetation were developing. Such regions 

 appear to have existed over part of the present southern Appalachian re- 

 gion at the beginning of Upper Devonic time — existed, in fact, during 

 Middle Devonic time as well. The rivers which washed these muds into 

 the Genesee estuary, which covered part of Pennsylvania and New York, 

 also brought remains of the land vegetation in the form of tree trunks 

 and spores. 



We have a modern example of such a type of deposit in the drainage 

 basin of the Vistula Eiver of Europe. This river drains the compara- 

 tively flat country of Poland and eastern Prussia, and carries its sediment 

 into the Bay of Dantzig, on the south coast of the Baltic. The salinity 

 of the waters of this bay is very low, being on the average only 7.22 per 

 mille for the surface and 11.66 per mille at a depth of 105 meters. The 

 marine life of this bay is of a limited and depauperate type, with a dwarf- 

 ing of those euryhaline individuals which are found. According to G. 

 Bishof,^ the mud brought down by the Vistula loses 23.3 per cent on igni- 

 tion, most of this being organic material. As deposited on the floor of the 

 bay, the mud is of such a deep black color that it is locally called pitch. 

 It covers an area of 615 square miles on the bottom of the bay. 



The Genesee estuary was apparently flooded by an advance of the ocean 

 waters from the north after the Genesee muds had been deposited. This 

 is indicated by the increase in the frequency of marine organisms^in the 

 succeeding West River and Middlesex shales. The muds were still carried 

 in by the rivers, but the salinity seems to have increased considerably. Of 

 especial interest is the sudden precipitation of millions of pelagic shells 

 of Styliolma fissurella, which accumulated in such quantities as to form 

 limestone beds often 6 inches or more in thickness and of great east and 

 west extent. Dr. John M. (Jlarke has estimated that this limestone some- 

 times contains as many as 40,000 individual shells to the cubic inch. 

 This enormous precipitation was apparently brought about by the influx 

 of the plankton-bearing currents into the brackish estuaries and the con- 

 sequent killing of these stenohaline organisms by the millions. The in- 

 tercalated black shales and the presence of plant remains and of plates 



' Lehrbuch der Chem. u. phys. Geologie 



