OSCILLATORY CHARACTER OF CONTINENTAL SEAS ^25 



generally wanting in the basin north of the Maryland-Pennsylvania line, 

 and (3) a final, more extensive submergence, bringing in Caryocaris, 

 Triarthrus, and Trinucleus. (See figure 3-1.) 



Chamhersburg limestone in the Mercersburg {Pennsylvania) hands — 

 Character and stratigraphic relations. — Sections of the Chamhersburg 

 limestone belts west of the Chambersburg-Greencastle-Martinsburg- 

 Strasbnrg band, while differing considerably among themselves, agree in 

 certain dominant features, wherein they differ from the more eastern 

 phase of the formation above described. It will be remembered that in 

 the eastern belt the Chamhersburg includes four easily distinguishable 

 faunal zones, namel}^, (1) Tetradium cellulosum, (2) Echinosph^erites, 

 (3) Mdulites, and (4) Christiania, followed in order by the Sinuites, 

 Corynoides, and Trinucleus zones of the Martinsburg. Further, that the 

 formation there begins often with the lower Echinosphserites bed, but 

 locally, as west of Marion and near Greencastle, Pennsylvania, and at 

 Strasburg, Virginia, 100 to 200 feet of compact Lowville limestone is 

 intercalated between that zone and the top of the underlying upper 

 Stones Elver. In the Mercersburg bands, as the more western repre- 

 sentation of the formation may be conveniently called, the same zones, 

 except the Christiania, are recognized, the first and the second, however, 

 more definitely than the third (Nidulites). A striking difference is 

 that the Lowville Tetradium cellulosum zone, instead of 1)eing at the 

 base, is here above the middle of the formation, the lower 100 to 175 

 feet being made up of a subcrystalline limestone containing a fauna 

 requiring its correlation with the upper Chazy of ISTew York, the Murat 

 limestone of Virginia, and the Hols ton marble of Tennessee. It is 

 to be noted further that even with this additional lower member tlie 

 whole formation is much thinner than in the Chambersburg-Massanutten 

 basin. Eegarding the thickness, there is to be added that in the middle 

 belts of the Mercersburg basin all the members are thicker than in the 

 belts to the east and, more especially, those to the west. In the most 

 westerly band both the Nidulites as well as the Christiania zone may be 

 absent entirely. 



The Lowville horizon, which in the Appalachian as well as the more 

 interior regions is nearly always marked by Tetradium, cellulosum, is com- 

 monly recognizable in the Chamhersburg limestone bands of the Mercers- 

 burg area. This horizon is well exposed in a railroad cut just south of 

 Fort Loudon, but the succession of the beds is somewhat confused by 

 faulting. The rather coarsely crystalline Caryocystites bed, which under- 

 lies the Lowville, contains, aside from a few species peculiar to the bed, a 

 disconcerting mixture of upper Chazy and Lowville-Black Hiver fossils. 



