The problem of the depositional environment of the bituminous shales is attacked by means of an analysis of the evidence for and against a fondo (relatively deep-water) origin of that great body of black, bituminous shale to which the names Marcellus, Genesee, Ohio, Sunbury, New Albany Chattanooga, and several others have been applied.

Attention is called to many features which indicate deposition in relatively deep, quiet, toxic water, as well as to others definitely opposed to a current concept that the shales were formed in very shallow water near shore by reworking of residual soils on a peneplain by the waves of a transgressing sea.

Among the features indicating a fondo environment are: the very nature of the material, with its large content of light-weight organic material which would not ordinarily come permanently to rest in any but quiet water; widespread beds of phosphate nodules; phosphatic material and a lag concentrate of miscellaneous marine debris such as conodonts, fish plates, et cetera, in the lower few inches of the shales; very thin, evenly stratified beds of siliceous and partly calcareous material, probably representing falls of volcanic ash, that are too evenly bedded and too widespread to have been subjected to wave action or to other disturbing influences; and faunal features indicating pelagic rather than near-shore conditions.

One of the most important features strongly opposed to a shallow-water, near-shore environment is the lack of evidence of wave action, such as should exist and should be unmistakable if the materials had been deposited in shallow water near shore. Further, the contact of the bituminous shales with the underlying rock fails to show important features which should have been produced if a sea had advanced over a peneplaned land surface.

It is suggested that these bituminous shales were laid down in poorly aerated water in the deepest remaining unfilled part of the Appalachian geosyncline and in those parts of the interior sea adjacent to it on the west and northwest, and that the black shales as they were deposited were being encroached upon by clino deposits, at least along the eastern and southeastern side of the water body. During the filling of the trough from the east and southeast, the coarser clastic sediments seem to have built out a delta-like shelf (undaform) bordered by a foreset slope (clinoform) leading down to the deeper part of the basin (fondoform). Normally, the undaform and clinoform seem to have encroached gradually westward over the fondo black shales, but at intervals, perhaps because of more rap d sinking or lessened sediment supply, black shale deposition extended farther landward over previously deposited sandy strata. Thus several bituminous shale units in the eastern part of the Appalachian trough, separated by units of sandy sediments whose combined thickness is several thousand feet, are believed to converge westward into a single continuous, but relatively thin, body of organic shales having a great range in age.