50 The Middle Devonian Deposits of Maryland 



of vciy dark colored thin liinet^tono. Tlie litholof^ical characters of these 

 shales agree closely with tliose of typical exposures of the Marcellus shales 

 in New York state. 



Fauna. — This member contains a meager fauna comprising but few 

 species and individuals. The most al)uiulant forms are Liorhynchns linii- 

 tare, L. myma, and SlijlloUnn pssureUa, which are common in the ]\Iar- 

 cellus formation of Xew York. 



Onondaga-Marcellus Boundary. — The contrast between the lithology 

 of the Onondaga and Marcellus shale members of the Romney is generally 

 well marked except near the zone of transition between the two. Black 

 bituminous shales are sometimes conspicuous elements of the lower mem- 

 ber but almost invariably form a subordinate part of the beds and are less 

 frequently fissile than those of the overlying Marcellus. Carbonaceous 

 shales comprise a large part if not the whole of the Marcellus member in 

 most sections and are highly fissile. The intergrading of the two types of 

 sediment will, however, always make it difficult in many sections to decide 

 upon a precise plane of separation. This intergrading is of much the 

 same character as that between the Marcellus and Hamilton in New York. 

 In some sections, as at 21st Bridge on the Potomac River, a band of 

 argillaceous limestone terminates the Onondaga shale member. 



Hainil 1)1)1 McuiJicr 



Character and Thickness. — This member of the Romney formation 

 has an approximate thickness of 1000 feet and is composed of shales and 

 sandstones. In recent exposures the shales, genei-allv bluish or bluish- 

 gray in color, vary in composition from rather coarse arenaceous to those 

 that are tine and argillaceous. The sandstones, which on fresh surface 

 are generally blue or gray in color, are not very coarse in texture and the 

 layers are often less than a foot thick. All of these rocks, however, on 

 long exposure usually present along the highway a slightly greenish or 

 yellowish-gray tint. Two or more prominent sandstone zones varying in 

 thickness from about 30 to 75 feet occur in this member of the formation. 

 The lower one is from 850 to 1050 feet above the base of the formation, in 

 the sections east of Wills Mountain, while the upper zone is at or near the 



