952 A. W. GRABAU THE TULLY LIMESTONE 



In Allegany and Washington counties, Maryland, black shale deposi- 

 tion began in Onondaga-Marcellus time, continuing throughout the Ham- 

 ilton with little change. This constitutes the Eomney formation, whicli 

 has been subdivided into an Onondaga member of dark shales and thin 

 limestones at the base, 100 to 150 feet thick, a Marcellus member of black 

 fissile shales about 500 feet thick in the middle, and a Hamilton member 

 at the top. This last is about 1,000 feet thick, and is composed of dark 

 shales often somewhat sandy, and carries several heavy sandstones in its 

 upper portion. 



In Allegany County these upper Eomney beds are abruptly succeeded 

 by black fissile shales about 90 feet in thickness, which carry a Naples 

 fauna, comprising Buchiola retrostriata, Fterochcenia fragilis, Styliolina 

 fissurella, Bactrites acimdus, etcetera.^ This formation has been corre- 

 lated with the Genesee of New York, but probably does not represent the 

 typical Genesee. It may, however, represent the West Eiver shales of 

 New York which overli-e the Gennundewa limestone or the Middlesex 

 black shales which succeed these. Farther east in Washington County, 

 Maryland, these black shales are wanting, and the olive shales and sand- 

 stones of the higher Jennings formation rest directly upon the Eomney. 



Origin of the Genesee Shale 



From what has so far been given, it is clear that the source of the sedi- 

 ment which composes the Genesee formation was to the south, and that 

 the thicker series of these beds in Pennsylvania point to continuous mud 

 deposition there, while the calcareous sediments, which formed the Tully 

 limestone, were accumulating over much of New York State. The in- 

 creasing spread of the black muds from the south finally put an end to 

 deposition of calcareous muds from the north, and thus black Genesee 

 shale came to overlie the Tully limestone. How far beyond the present 

 northern line of outcrop the Genesee sedimentation extended, whether it 

 reached the region of the reefs which furnished the lime mud of the Tully 

 limestone, will forever remain an unsolved problem, since these reefs, if 

 such they were, liave been wholly removed by post-Paleozoic peneplana-- 

 tion. Their position was probably somewhere in Canada north of Lake 

 Ontario. 



From the relation of the Genesee sediments to the other formations, it 

 is apparent that these muds could have been supplied only by a river 

 which entered the Upper Devonic sea from the south. It is probable thai 

 Maryland was at this time above water, but so low that little erosion was 



•'Maryland Oeologjical Survey. Miflflle and Upper Devonian, 1913, p. 347, 



