WILLIAMS.] 



THE APPALACHIAN REGION. 



269 



however, no beds lower than the Ohazy-TrentoD horizon haveasyel been 

 certainly identified upon paleoiitologica] evidence, while at the top of 

 the scries the Permian strata are wanting. Between these limits the 



series is quite complete, although many of the members are, in com- 

 parison with their Pennsylvanian equivalents, considerably attenu- 

 ated. The formations distinguishable along the Potomac section are 

 for the most part the same as those recognized by the Pennsylvania 

 geological survey. The names and numerals by which these are usually 

 designated are given, for convenience of reference, in the following table: 



No. New York and Pennsylvania names. Maryland and Virginia equivalents. 



o 



I I 



I 1 



( 



XV 

 XIV 

 XIII 

 XII 



XI 

 X 



IX 

 VIII 

 VII 



VI 

 V 

 IV 



III 

 II 





) l Pittsburg scries. 



Productive Coal Measures Barren Coal Measures. 



) ( Alleghany River series. 



Pottsvilie conglomerate (Millstone Great conglomerate, 

 grit). 



( Mauch Chunk red shales i .. , , , 



,, , • ,. . dreenbrier sua es. 



/ Mountain limestone s ■, 



Pocono sandstone (Vespertine) Montgomery grits. 



Catskill sandstone Eampshire. 



(Chemung i , . ,, 



I Hamilton (Mareellus, shales .... s •'<'-n l n-s-Iu,umey. 



< )riskauy sandstone Monterey. 



( Lower Helderberg limestone ) ., . , 



jSalina group (sandstone) \ ( , ' ,,1, ' nt rock - 



s Niagara limestone. 



I ( linton shales Roe k wood. 



S Medina sandstone ) A , . . , . 



.. -, , . , Massanu ten sandstone. 



/ Oneida conglomerate s 



Hudson River shales Mart Insburg shale. 



Trenton-Chazy limestone Shenandoah limestone. 



•' Valley limestoue." 

 Potsdam sandstone 



This thick succession of conglomerates, sandstones, shales, and lime- 

 stones accumulated as a part of the deposits of the vast sea which, 

 during Paleozoic times, occupied the interior of the North American 

 continent. Since these formations are so much thicker in the Appa- 

 lachian belt than in the Mississippi basin they must, as has been shown 

 by Hall" and Dana," have been deposited in a trough which was 

 undergoing a gradual depression. Their aggregate thickness in Penn- 

 sylvania is estimated at 10,000 feet. There is every evidence that 

 this vast deposition took place from the east toward the west, and we 

 are obliged to assume as the source of supply for so great an amount 

 of material, a continental mass over and beyond what is now the 

 Coastal plain. It is not improbable that the more crystalline portion 

 of the Piedmont plateau may represent a remainder of this eroded and 

 sunken continent. The conditions of accumulations through Paleozoic 

 times were probably quite similar to what they have been and still are 



