OBCILLATORY CHARACTER OF CONTINENTAL SEAS 327 



described, we find in an exposure of a western belt at the Blue Spring, 3 

 miles southwest of Mercersburg, the usual subcrystalline limestone at the 

 base, terminating above with the Caryocystites bed. This is succeeded by 

 a Lowville-Black Eiver representative only 2 to 4 feet thick. Several of 

 the lower layers of this bed are crowded with Beatricea gracilis, while 

 others above are filled with Leper ditia. Next comes 12 feet of granular 

 uneven-bedded dark gray or blue limestone, representing the Echino- 

 sphserites bed, then about 55 feet of earthy thin-bedded, compact dark- 

 blue limestone, weathering cobbly in lower half and shaly in upper, and 

 in which organic remains are very scarce. This is followed by drab cal- 

 careous shale. In Licking Cove, 2 miles farther southwest, the section is 

 essentially the same except that the upper beds are much better exposed. 

 In the absence of fossils it is very difficult to decide whether the 55-foot 

 bed is really the correlative of the 36-foot bed at Fort Loudon. If it is, 

 then shale deposition began here earlier than usual in the Chambersburg 

 limestone belts. More likely it corresponds to the 40-foot bed overlying 

 the top of the Chambersburg in the Fort Loudon section, and hence that 

 the Nidulites and Sinuites zones are wanting in the Blue Spring and 

 Licking Cove sections. 



The Dickeys, Pennsylvania, section. — A good exposure of the middle 

 beds of the formation occurs 4 or 5 miles southeast across the strike from 

 Fort Loudon in a railroad cut just north of Dickeys station. Overlying 

 the Caryocystites zone, which is easily recognized, is a bed 34 feet thick, 

 of massive, light to dark bluish gray, compact limestone containing Bea- 

 tricea and Tetradium celhdosum in abundance. As the Leperditia bed 

 does not occur in this section, and the following beds seem to be referable 

 to the Nidulites zone, it would appear that this 34-foot bed represents 

 chiefly the hiatus at the base of the Lowville in the Fort Loudon section. 

 Over it in the Dickeys cut come 85 feet of bluish gray fine-grained lime- 

 stone, rather thin and irregularly bedded, and weathering cobbly in the 

 lower and upper parts. The middle part is comparatively heavy bedded 

 and mottled with clay, giving the weathered surface a reticulated aspect. 

 Although the most characteristic fossil was not seen — indeed it has not 

 been observed in any of the Mercersburg bands — this bed is correlated 

 with the Nidulites zone because its upper part contains a number of fos- 

 sils so far found only in association with Nidulites favus in the Massa- 

 nutten basin. The Echinosphaerites horizon may be included in the lower 

 part but was not recognized. Next comes a massive bed of dark bluish 

 gray crinoidal limestone, 6 feet thick, containing characteristic fossils of 

 the Sinuites bed. The overlying beds are not well shown, but appear to 

 be in general character as in the Saint Thomas section next described. 



