322 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



a brachiopod and bryozoan fauna highly suggestive of the upper part 

 (Phylloporina bed) of the "Black Eiver shales" or Decorah formation 

 of Minnesota and Iowa. 



The lower Echinosphaerites bed is overlain by the Nidnlites zone. This 

 averages approximately 200 feet in thickness, but readies nearly 300 feet 

 in the vicinity of Greencastle and Marion, Pennsylvania, where an un- 

 usual amount of fine clastic matter in most of the beds is partly respon- 

 sible for the great thickness of the Chambersburg at these localities. As 

 a rule the Mdulites bed contains the heaviest ledges in the formation; 

 but again the Greencastle section includes equally thick, or even heavier, 

 beds near the top of the formation that elsewhere in the basin seem to be 

 absent entirely. The Nidulites bed was easily recognized by its charac- 

 teristic fossil in every full section of the formation observed in this basin. 

 It and the overlying Christiania bed, together more tlian 600 feet thick, 

 represent an age in the general time scale between the top of the Black 

 Eiver and the base of the Trenton, in New York. 



Following the Nidulites bed, the section again shows great local varia- 

 tions. At Chambersburg the remaining upper part of the formation 

 consists of limestone ledges alternating with generally thicker beds of 

 shale, the whole belonging to the Christiania bed and aggregating ap- 

 proximately 270 feet. In a few inches of rock at. the extreme top occurs 

 a rather poor representation of the Sinuites fauna, which invaded from 

 the west, and a few feet beneath this the upper Echinosphgerites and 

 principal Christiania zone. These upper fossil zones are still recogniza- 

 ble 6 to 8 miles up the valley. In the opposite direction, say 2 to 6 

 miles southwest of Chambersburg, they are especially rich in organic 

 remains. Farther south, however, the fossil zones are gradually ob- 

 scured in the great expansion of the upper beds of the formation already 

 noted as occurring in the vicinity of Greencastle. Here the beds above 

 the Nidulites zone and to the base of the Martinsburg shale vary in 

 thickness between 300 feet and fully 500 feet. Very heavy beds of dark 

 gray, sparsely fossiliferous limestone, weathering chalky and locally more 

 or less arenaceous, make up the greater part of the upper 100 to 200 feet 

 in the thickest sections. At Pinesburg, Maryland, the corresponding 

 portion of the formation is thinner and consists mostly of shale, the 

 general aspect and composition of the whole formation at this point 

 being essentially the same as at Chambersburg itself. At Martinsburg, 

 West Virginia, where the Lowville member is absent, the lower Echino- 

 sph^rites bed, 15 to 20 feet thick, and the Nidulites bed, 300 feet thick, 

 the next or uppermost bed of the formation (Christiania zone) is only 

 about 20 feet tbick. At Strasburg, Virginia, the N'idulites bed, a solid 



