132 TRANSACTIONS OF THE [jUNE 6, 



Notes on the Geology of Skunnemunk Mountain, Orange 

 County, New York.^ 



BY CHARLES S. PROSSER. 

 [Published by permission of the Director.of the U. S. Geological Survey.] 



The geologic age of the rocks composing the Skunnemunk and 

 Bellvale mountains in Orange County, New York, as well as their 

 continuation in New Jersey under the names of the Bearfort, 

 Kanouse, Copperas, and Green Pond mountains, has long been a 

 question of great uncertainty. The complex geologic structure of 

 the region, in connection with the almost total absence of fossils, 

 has made it very difficult to assign these rocks to their proper 

 position in the geological column. On this account a concise sketch 

 of the various opinions that have been advanced as to the geologic 

 age of this terrane may be of some general interest. 



One of the earliest references to these rocks is contained in Pro- 

 fessor Henry D. Rogers's report of 1836, where it is stated that 

 under certain conditions " it becomes a little difficult to know it 

 [the Green Pond Mountain conglomerate], at first sight, from the 

 less calcareous varieties of the Potomac marble conglomerate 

 [Triassic breccia]. In fact, it is possible that the two may be of 

 coeval formation."" 



The next reference is contained in the report of W. Horton " On 

 the Geology of Orange County," New York, to W. W. Mather in 

 1839. It was stated that " Bellvale Mountain extends quite across 

 the town of Warwick. The southeast side, the top, and about one- 

 third of the descent on the northwest side are composed of gray- 

 wacke and graywacke slate, standing nearly vertical. Its colors 

 are gray, greenish- and bluish-gray, and brick-red 



" Skunnemunk is similar to Bellvale Mountain, passing unbroken 

 nearly across Monroe. Its southeast side, top, and part of north- 

 west side are graywacke in all the same varieties."* The geologic 

 age was not stated. 



Professor Henry D. Rogers was inclined to consider the New 

 Jersey portion of this terrane as of Middle Secondary age, the 

 name that he used for the Triassic red sandstone to which the 

 name Newark group was given by Mr. W. C. Redfield.* Professor 

 Rogers said: " From the wholly insulated position of these uncon- 

 formable strata, encompassed on all sides by rocks of older date, 

 and from the non-appearance hitherto of any organic remains of a 

 distinctive kind, by which to infer their age and mode of origin, we 

 are prevented from arriving at any very satisfactory views regarding 



1 The drawing illustrating this article has been lost, and could not be re- 

 produced in time to appear. 



2 Rep. on the Geol. Surv. of New Jersey, p. 130. 



3 N. Y. Geol. Survey, 3d Ann. Rept., 1839, p. 146. 



* Am. Jour. Science, 1st ser., vol. xxii, p. 357. See Prof. I. C. Russell on 

 " Tlie Newark System," Am. Geologist, vol. iii, pp. 178-182. 



