Marylaxd Geological Survey 39 



Straits, while the southern faunas are distinctly of Gulf of Mexico origin. 



The small Appalachian Sea of Lower Devonian time varied considerably 

 in area. The later Silurian fonnations extend to Tennessee, but the New 

 Scotland is not known south of central Virginia. Toward the close of the 

 latter formation this trough was also much reduced in width, since the 

 Becraft limestone is known only in the eastern portion, from Virginia to 

 the Helderbergs. The reduction was continued into the early Oriskany, 

 as south of Pennsylvania the latter series rests in the western part of the 

 trough on different horizons of the eroded Helderberg. 



The Oriskany Map (Plate VII). — Very early in Oriskany time the re- 

 stricted sea in the eastern Appalachian trough again spread westward 

 and attained to southern New York, which had been completely drained 

 in the Helderberg Mountain area after the Becraft of Helderberg age. 

 These early Oriskany faunas are direct developments from the previous 

 Helderberg elements of southern Atlantic derivation. With the exten- 

 sion of this increasing sea, evolution of marine life became rapid, and 

 the record is probably more complete in Maryland ajid the adjoining states 

 than elsewhere. Toward the end of the Oriskany the sea gi-ew shallow 

 south of New York, more North Atlantic forms (Gaspe) appeared, and 

 finally the waters were withdrawn for a time from the southern 

 Appalachian trough. North of Pennsylvania the late Oriskany faunas 

 were continued in those of the true Oriskany sandstone and the Esopus 

 grit. The New York Basin at last moved westward through New York, 

 and the close of the eastern Oriskany series is seen in the Decewville for- 

 mation of Ontario. These local differences in the strata, their varying 

 position, and the accumulation of sandstones and black cherts bear wit- 

 ness to the unrest of Appalachia and to its elevation during the later 

 portion of the Lower Devonian. The trough south of New York then re- 

 mained land from the latest Oriskany until the late Onondaga of Middle 

 Devonian time. While at this time Appalachia was not much elevated, 

 Acadia began to undergo more and more severe crushing, with considerable 

 mountain making. 



The southern or Gulf cmbayment presents in the Camden formation 

 only the latest stage of the Oriskany. This fauna, which continues un- 



