Maryland Geological Survey 113 



f'unii,slie(l t(j tlio interior Devcdiinn sea of the Appalachian region, between 

 the beginning of Middle Devoniaji time and the close of the Devonian, a 

 mass of terrigenous sediments which, if restored upon a sea-level plain of 

 Appalachia, " would constitute a mountain range closely resembling in 

 height, extent, and mass the Sierra Nevada of California." ' In marked 

 contrast with this great thickness of clastic sediments which represent 

 Devonian time in tlie Alleghany region we find in tlie Ohio valley less 

 than 200 feet of sediments representing about the same time interval 

 which is represented by the 5000 feet of Devonian sediments in the 

 Middle Alleghany region. Although the land which furnished most of 

 the Devonian sediments of the Middle Alleghany region has long since 

 disappeared, we know from the great mass of sandy and argillaceous sedi- 

 ments which it furnished that it was composed chiefly of noncalcareous 

 rocks. In the Ohio and Mississippi valleys we know both from, the 

 character of the sediments and the remnants of Devonian lands yet 

 uncovered in the region that the source of sediments during Middle De- 

 vonian time in the western part of the Appalachian Gulf was chiefly 

 limestone lands representing Silurian and Ordovician terranes. Such 

 limestone lands formed the western shore of the Onondaga sea in Ohio, 

 Indiana, and Kentucky. It is doubtless due chiefly to the different 

 constituents of the rocks which furnished sediments to the Middle De- 

 vonian sea on the eastern and western sides of the Api>alachian Gulf that 

 we find limestones representing the sediments of Onondaga and Hamilton 

 time in the central states, and shales and sandstones their chief constitu- 

 ents in the Marvdand region. 



These contrasts in the type of Middle Devonian sediments on opposite 

 sides of the Appalachian Gulf are comparable with those which we find 

 at present around the coasts of the Gulf of Mexico. Otf the mouth of 

 the Mississippi Eiver immense deposits of argillaceous mud are fonning 

 which may eventually become a shale formation not unlike the Eomney 

 of Maryland. A few hundred miles to the eastward in the Florida-Bahama 

 region we find vast areas of chalky mud from which we nuiy expect a lime- 



'Md. Geol. Survey, Vol. IV, Pt. I, p. 62. 

 8 



