230 INDEX TO THE STEATIGRAPHY OF NORTH AMERICA. 



lying Helderberg member, that, for this region at least, it would be doing violence to the natural 

 classification of the rocks to draw a line of systemic importance between them. According 

 to a classification taking into account both the life history and structure of the strata, this 

 divisional line should be drawn either between the flinty beds at the top of the Hunton limestone 

 and the base of the black shale and chert of the overlying Woodford formation, or between the 

 Niagara and Helderberg members. In the latter case the lower member only would be Silurian, 

 and the two upper members Devonian. 



I-J 16-17. ALABAMA, GEORGIA, TENNESSEE, WEST VIRGINIA, AND VIRGINIA. 



In the zone of Appalachian folds from Maryland southward, the Silurian strata . 

 consist of the basal littoral formations and the overlying Clinton ("Rockwood" or 

 "Dyestone") of the Southern States. The basal sandstones (Bays, Ordovician, 

 and Clinch, Silurian) extend from Virginia along the valley of east Tennessee to 

 the longitude of Knoxville. The Clinton or "Rockwood" alone is continuous to 

 Alabama. Higher Silurian deposits, the equivalents of the Niagaran and Cayugan, 

 thin and disappear in southwestern Virginia and northeastern Tennessee, as shown 

 in the sections given below. The Helderbergian is here classified as Devonian, and 

 the upper limit of the Silurian therefore falls in the lower part of the Lewistown 

 limestone of the following sections. The Silurian outcrops in narrow belts inti- 

 mately associated- with and practically coextensive with -the Devonian and is, in 

 view of the small scale of the map, indistinguishable from the Devonian in the 

 generalized drawing of the complex structure. The Silurian is therefore represented 

 with the Devonian by the color of the latter south of northern Virginia. 



In eastern West Virginia (Piedmont quadrangle) the Silurian has been classi- 

 fied by Darton ^^^ as follows : 



Top. Feet. 



Monterey sandstone, in part; massive calcareous blue-gray sandstone (in part Devonian). 215-300 

 Lewistown limestone (equivalent to Lower Helderberg [Devonian], Salina, and Niagara): 



Clierty limestone (Lewistown chert lentil) 130-300 



Massive light blue gray limestone 20- 8.0 



Shaly limestone 100 



Dark-colored limestone 30- 60 



Flaggy limestone 450-980 



Thin beds of impure limestone with alternations of greenish and gray calcareous shale. 250-310 

 Eockwood formation (Clinton): 



Gray sandstone 12- 20 



Gray, brownish, and greenish shale with iron ore and thin limestone beds 525-750 



Cacapon sandstone: Red sandstone, mainly thin bedded 300 



Tuscarora quartzite: White or gray massive quartzite 480 



Juniata formation : Brownish-red sandstone and shale 75b-|- 



The Juniata, Tuscarora, and Cacapon Darton regarded as the representatives 

 of the Medina of New York. 



Traced through the Franklin quadrangle, south of the Piedmont, the divisions 

 of the Silurian in the Staunton quadrangle, in central Virginia, are similar. but 

 reduced in thickness, as shown by Darton ^^''* in the following section: 



Silurian rocks in Staunton quadrangle, Virginia. 



Feet. 



Monterey sandstone (Silurian and Devonian, Oriskany), in part calcareous 0-300 



Lewistown limestone (Silurian) : Cherty limestone, pure limestone 300-500 



Rockwood formation (Clinton): Gray quartzite, sandy shale 150-200 



Massanutten sandstone (Medina) : Reddish sandstone, gray quartzite, red and gray sandstones. 500-600 



