Lower Helderberg Rods of Port Jervis, etc. 297 



ates : but subsequent investigation by myself, and the au- 

 thoritative identification by Prof. Hall of the hard, calcareous 

 bottom layer (5a) as Upper Pentamerus, has convinced me 

 of my error. 



From the top of Trilobite Ridge to the foot of the Cauda 

 Galli Ridge north-west of it, Oriskany fossils predominate. 

 There is, however, such a gradual shading off from one into 

 the other, that no one whose knowledge of the Lower 

 Helderberg and Oriskany strata had been acquired by the 

 study of their exposures in this locality, would ever think of 

 running the line separating the Silurian and Devonian ages, 

 between the two. They seem so intimately blended that the 

 exact line between them is an arbitrary one altogether. 

 Thanks to Professors Hall and Dana, we now have here in 

 the mural south-east front of the Cauda Galli, as plain a 

 dividing Avail between the Silurian and Devonian ages, as 

 there is between the Upper and Lower Silurian east of it. 



From the Oriskany (No. 6), the following species have 

 been determined : — * Tentaculites elonyalus, *Platycera& Geb- 

 harclii, Platyostoma ventricosum, *l > leri?iea lextilis (var. 

 areiiariii), * Pensselaeria ovoides, Eatoniu peculiar is, *8piri' 

 for arrectus, /S. arenosus, Meristella sp. 



We find therefore at this point, a total thickness of some 

 five hundred feet of well-marked Lower Helderberg rocks, 

 overlaid to the north-west by an immense development of 

 Cauda Galli Grit. The strata are not all visible at any one 

 place ; but the complete series is given from several quarries 

 in the immediate vicinity of the town. The main trend of 

 the monoclinal ridges that characterize the region, is usually 

 about N. 60° E. and S. 60° W. ; but they are cross,, I ob- 

 liquely by transverse flexures, running nearly north and 

 south, which elevate the strata in anticlinals transverse to the 

 main uplift. In these are located the quarries that furnish 

 the best sections; the particular niembers visible in each, 

 depending, of course, on the amount of transverse uplift 

 from below, and the extent of erosion above. 



