240 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The overlying Helderberg limestone is assigned by Prosser to the Devonian. 

 The Silurian of New Jersey has also been described by KiknmeF"'' as follows: 



Contrary to long prevalent and apparently weU-established belief, the lower and middle, 

 portions of the Silurian system are not represented in New Jersey. Their absence in this and 

 adjoining regions is indicative of somewhat widespread earth movements unaccompanied in 

 this region by folding, which closed the period of deposition indicated by the Martinsburg 

 sedunents or possible overlying beds afterwards removed by erosion and raised the region 

 above the zone of sedimentation. When deposition began again, late in Silurian time, beds of 

 coarse conglomerate were laid down, followed by sandstones, shales, and limestones, the earlier 

 sediments being those of a low-grade delta in an arm of the Appalachian gulf." These condi- 

 tions of deposition prevailed with but slight changes of elevation into Devonian time. 



The Shawangunk conglomerate (the Oneida conglomerate of many previous publications) 

 ie chiefly a coarse quartzite and conglomerate composed of small white-quartz pebbles embedded 

 in a siliceous matrix. Its color is generally steel-blue, but some beds have a yellowish tinge 

 and reddish layers occur near the top. Layers of black shale a few inches in tluckness are 

 locally intercalated between thick beds of conglomerate and grit. Between this formation 

 and the Martinsburg shale there is a gap representing the upper part of the Ordovician and aU 

 of the Silurian below the Salina of the fuU New York section, but there is no marked divergence 

 of dip and strike where the two formations outcrop in proximity, and the actual contact is 

 nowhere exposed in New Jersey. The beds overlying the Shawangunk conglomerate are red 

 sandstone and shale, and the transition from the Shawangunk is made through a series of 

 alternating red sandstone and gray conglomerate, so that the upper limit of the Shawangunk is 

 not sharply defined. Its thickness is probably from 1,500 to 1,600 feet. 



So far as known, the formation is barren of fossils in New Jersey, but at OtisviUe, N. Y., 

 a eurypterid fauna has been found in the black shale intercalated with the conglomerate. In 

 the OtisviUe section tliis fauna, which elsewhere appears only and briefly at the base of the 

 Salina, repeats itself many times through a thickness of 650 feet." The Shawangunk con- 

 glomerate is foUowed by 2,500 feet or more of shales and limestones also referable to the Salina; 

 hence for this region it represents only the lower portion of that group. 



The red sandstone and shale which immediately overlie the Shawangunk conglomerate 

 have until recently been regarded as the equivalent of the Medina sandstone of New York and 

 have been so called, but, for the reasons just cited, it is evident that they are much younger 

 than Medina and that they must be included in the Salina group. Moreover, they lie some 

 distance below a limestone which is correlated with the CobleskiU of the New York section. 

 The name High Falls has been apphed to the red shales that overlie the Shawangunk con- 

 glomerate in Ulster County, N. Y., and has been adopted for New Jersey in place of Medina, 

 which is not applicable. 



The lower beds consist of a hard red quartzitic sandstone, intercalated with some green or 

 gray sandstones and softer red shales which become more abundant in the upper part of the 

 formation. The formation has an estimated thickness of 2,300 feet at Delaware Water Gap. 

 It is not known to contain fossils, but its age is fixed by its stratigraphic position. 



The formation known as the Green Pond conglomerate occurs in an isolated belt of Paleozoic 

 rocks which extends through the middle of the pre-Cambrian Highlands of New Jersey. In 

 constitution it is similar to the Shawangunk conglomerate, with which it is correlated, but, 

 inasmuch as it is stdl an open question whether the Paleozoic strata of the Green Pond Mountain 

 region were once continuous with the great mass of Paleozoic sediments which lie some distance 

 to the northwest, or whether the Green Pond region represents a separate basin shut off on the 

 northwest from the large Paleozoic sea although communicating with it to the northeast, it 

 has seemed best to retain for the present at least both Shawangunk and Green Pond as names 

 for these conglomerates in their respective fields. 



a Clarke, J. M., Bull. New York State Mus. No. 107, 1907, p. 303. 



