112 NEW YORK STATE MUSEUM 



no such formation could be accounted for without complete weather- 

 ing of all rock and the removal of everything except the quartz. If 

 done in this way it is difficult to accovmt for the rounded condition 

 of the grains and the extreme purity and the failure to find any con- 

 glomerate facies. 



Perhaps the most consistent explanation is the assumption of an 

 intermediate source, that is, a series of sedimentary formations, 

 formerly covering the gneisses which has been destroyed in the 

 making of the Poughquag and its associates. Such a formation, of 

 course, if it carried sandstone or quartzite, would be capable of fur- 

 nishing supplies consistent with the development of such formation 

 as the Poughquag. The Manhattan-Inwood-Lowerre series might 

 meet such conditions. It is particularly illuminating, therefore, to 

 observe that the coarser members of the Hudson River series are 

 made up chiefly of lithic grains instead of mineral fragments. In 

 other words, the grains are largely complex rock fragments including 

 fragments of slates, phyllites, schists, dolomites and quartzites. These 

 are all fragments of some older somewhat metamorphosed sediment- 

 ary series which was largely destroyed to furnish the sediments of 

 the Cambro-Ordovician. There is no escape from the conclusion 

 that such modified sediments were available because the fragments 

 can have no other interpretation. 



The only question, of course, is whether any remnants of such a 

 series are still to be seen. Unless the Manhattan-Inwood-Lowerre is 

 such a series, there is no hope of finding it, but the series mentioned 

 is quite competent to furnish just such supplies as are needed. The 

 present exposures are more completely metamorphosed than are 

 the grains found in the Hudson River series, but if one assumes 

 that in Cambrian time only the uppermost members were 

 exposed and that it was these portions which furnished the materials 

 for the sediments, this discrepancy does not seem at all disturbing. 

 It is entirely possible that there were sandstone members available 

 from which the Poughquag supplies were derived. It ought to be 

 expected also that the upper members of the Grenville floor, above 

 the present igneous intrusions in which pegmatites are so prominent, 

 had large developments of vein-quartz, representing the end products 

 of igneous injection. This would furnish large amounts of addi- 

 tional quartz when these overlying rocks were destroyed. 



Although there are few fossils, it appears that the Poughquag was 

 laid down under marine conditions, for fragments of Trilobites 

 have been found in this formation on the north margin of the High- 



