238 J. Barrell — Upper Devonian Delta of the 



The original color of the Skunnemunk quartzite pebbles, on the 

 contrary, was a grayish white to greenish gray, stained later to 

 a purplish or pinkish white, lighter in color value than the 

 matrix of the Green Pond conglomerate. The evidence for 

 this conclusion is as follows : 



The lower part of the Skunnemunk (conglomerate contains 

 beds of green flags and greenish conglomerates well shown 

 west of Hanks Pond and along the carriage road leading to 

 the summer residence of Mr. R. F. Cross. In this greenish 

 conglomerate the quartzite pebbles, the same in texture and 

 size as in the purple beds above, are grayish white or green- 

 ish gray. In the purple beds they are whitish pink or purple. 

 The vein-quartz pebbles are white in all cases, though stained 

 a little on the outside and on internal fracture surfaces. The 

 larger quartzite pebbles, especially in the beds only slightly 

 ferruginous, are pale in the center with a purple rind on the 

 outside. 



The logical explanation of these facts is based upon the 

 observation that white vein quartz is not porous and hence 

 does not stain readily by taking up solutions of iron in the 

 ground water. Quartzites, on the contrary, show various degrees 

 of porosity, from sandstones, the most porous of rocks, to 

 vitreous quartzites, almost without porosity. Ferruginous 

 solutions becoming oxidized in sands and gravels stain them 

 yellow, as seen in the yellow gravels of the Atlantic Coastal 

 Plain. This stain permeates the porous pebbles in direct ratio 

 to their porosity. Subsequent diagenesis in the ancient forma- 

 tions has eliminated the water from the iron oxide and turned 

 the yellow of limonite to the reddish brown or purple of 

 hematite, which if diluted by the rock substance becomes pink 

 or pale purple. In the Skunnemunk conglomerate, although 

 the matrix is of about the same depth of color as in the Green 

 Pond conglomerate, only a few pebbles attain this same depth 

 of stain, most of them remain distinctly lighter, and some of 

 the greenish, jaspery pebbles do not change in color except on 

 the surface. 



The Green Pond conglomerate, of the Silurian, rests uncon- 

 formably upon pre-Cambriau gneiss and Cambro-Ordovician 

 limestone, the basal Cambrian quartzite in New Jersey being 

 very thin and in many places wanting. The material of the 

 Silurian is apparently derived from deep erosion of these 

 gneisses at some distance to the southeast, sufficient to permit 

 the segregation and rounding by transportation of the white 

 vein quartz which is a minor constituent of the gneisses. 



In regard to the Skunnemunk conglomerate, of Upper 

 Devonian age, the evidence shows that most of it was not 

 derived from a local source ; neither the pre-Cambrian gneisses, 



