Appalachian Geosyncline. 237 



they measure about 2000 feet, but the strata there remain fos- 

 siliferous into Portage time.* The Bellvale flags grade in 

 turn into the massive Skunnemunk conglomerate with an 

 eroded top, but still 2500 feet in thickness in the deepest part 

 of the syncline. The broader structural evidence indicates no 

 local source for these conglomerates, since the tine grain and 

 fossiliferous character of the Hamilton beds gives no sugges- 

 tion of a mountain-walled inlet. No overlap suggests later 

 deformation, and the Skunnemunk conglomerate was separated 

 from the Green Pond conglomerate and the Archean by from 

 2500 to 3500 feet of Middle Devonian strata. 



Although no local basin existed, the evidence indicates that 

 continued upwarping of the land to the east exposed earlier 

 quartzites to erosion, for the pebbles in the Skunnemunk are 

 less abundantly white quartz, but principally of quartzite. As 

 seen in the southern end of Bearfort mountain in New Jersey, 

 the quartzite pebbles are dominant to the base. The section 

 of Skunnemunk mountain, however, 27 miles northeast shows 

 in the lower beds pebbles up to three inches in diameter, domi- 

 nantly of white quartz, but with some pale pink and gray 

 quartzite. The upper beds are much more abundantly con- 

 glomeratic, with quartzite pebbles as the dominant constituent 

 and ranging in size up to eight inches. All are well rolled, 

 prolate spheroids being the common form. 



According to Kiimmel and "Weller, many of these pebbles 

 are lithologically identical with the quartzite beds in the Green 

 Pond formation, and seem to have been derived from it. The 

 writer has observed internal evidence, however, which points 

 strongly to a different and somewhat distant source. The peb- 

 bles in the Green Pond conglomerate are overwhelmingly 

 white quartz. At the time of the deposition of the Upper 

 Devonian the Silurian conglomerate had not become buried 

 over 3000 feet nor subjected to mountain-making movements. 

 If it supplied the material for the upper formation the white 

 quartz pebbles would have had at that time greater resistance 

 and coherence than the sandstone matrix and should apparently 

 have contributed more conspicuously to the overlying con- 

 glomerate. Furthermore, the quartzite pebbles of the Skunne- 

 munk are well rolled, implying considerable wear before 

 deposition, and the grain of the quartzite pebbles is fine and 

 somewhat more vitreous than the matrix in the dominant beds 

 of the Green Pond conglomerate. A more positive indication 

 is derived however from color. The matrix of the Green 

 Pond conglomerate is a purple-brown with tinges of deep red, 

 the few lighter colored beds not being volumetrically important. 



* C. S. Prosser, The Devonian system of eastern Pennsylvania and New 

 York, Bull. 120, U. S. Geol. Surv., p. 12, 1894. 



