326 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Its age may be early Lowville, but for the present it is placed at the top 

 of the upper Chazy member. The contact between the Caryocystites bed 

 and the next following exhibits unmistakable evidence of interrii])t('d 

 sedimentation. The first 6 inches or so of the Lowville consists of irregu- 

 larly bedded, dark gray, rather compact clayey limestone, which fills 

 slight irregularities in the top surface of the lower bed. It is full of a 

 slender Beairicea, Helicotoma planulatoides, and other fossils, while para- 

 sitic bryozoa and the expanded bases of Cleiocrinus are attached to the 

 eroded surface of the bed beneath. All of the fossils suggest latc^ F.ow- 

 ville, but the Beatricea is of exceptional importance for correlation pur- 

 poses, as it is usually present in this area, and occurs also in the Cham- 

 plain Valley to the north and in central Alabama to the south in beds 

 regarded as of the same age. This thin basal deposit is followed at Fort 

 Loudon by 8 or 9 feet of bluish, subgranular and suboolitic limestone, 

 2ertain seams of which are filled with Leperditia and other ostracods. 

 Then comes an 8-foot bed of pure dove limestone, resembling oolite when 

 moistened, but really a very fine conglomerate. This contains Tetradium 

 cellulosum, Leperditia, and Helicotoma. It is the last of the undisturbed 

 sequence shown on the east side of the cut. In another near-by exposure 

 a darker, subgranular, massive limestone, 12 feet thick, overlies the dove 

 limestone, and is followed by about 15 feet of thin-bedded and shah- 

 layers corresponding to the displaced ledges shown on the west side of the 

 southern end of the cut. Fragments of the characteristic fossil of the 

 zone, Echinosphcerites, occur near the top with Cleiocrinus grandis. These 

 with other fossils procured from the last bed indicate a position between 

 the Black Eiver and the basal part of the Trenton limestone. 



Above the Echinosphserites bed the Fort Loudon section shows 36 feet 

 of fine grained, cobbly weathering dark limestone, correlated on litho- 

 logical grounds and because of its stratigraphic position with the Nidu- 

 lites bed. This is followed by 1 to 2 feet of black, partly shaly, crinoidal 

 limestone referred to the Sinuites bed. Here, however, the Sinuites is 

 rare, the most striking and commonest fossil being Trinucleus concen- 

 tricus. This, the first of the beds referred to the Martinsburg formation, 

 is succeeded by about 40 feet of hard, platy argillaceous limestone inter- 

 bedded with black slaty shale. The last bed is correlated with the argil- 

 laceous limestone and calcareous shale (Corynoides bed) overlying the 

 Christiania bed at Strasburg, Virginia, and Martinsburg, West Virginia, 

 at which places, however, it is much thicker — respectively 300 feet and 

 120 feet. 



Blue Spring section. — Comparing other sections of the Chambersburg 

 limestone in the Mercersburg bands with the Fort Loudoo section, above 



