IO NEW YORK STATE MUSEUM 



PRECAMBRIAN ROCKS 

 Grenville Series 



General character. The Grenville series of strata, including 

 possibly some contemporaneous igneous rocks, are considered to 

 belong among the oldest known, or Archeozoic, rocks of the earth. 

 These strata represent original shales, sandstones and limestones 

 which have become thoroughly crystallized into various schists and 

 gneisses, quartzites and crystalline limestone or marble. The 

 stratification is usually rather distinctly preserved though not with 

 its original sharpness. A more or less well-developed foliation is 

 always parallel to the stratification. 



Grenville rocks are not very prominent in the Schroon Lake 

 quadrangle, the combined definitely known areas totaling not over 

 12 square miles. It is quite certain, however, that Grenville strata 

 of great thickness once spread over not only the whole area of the 

 quadrangle and the 10,000 square miles of the Adirondack region, 

 but also over much of eastern Canada. They were, no doubt, 

 mostly deposited under marine waters much like the typical sedi- 

 ments of later ages. Within the quadrangle no positive proof for 

 great thickness has been obtained, but in other districts a thickness 

 of at least several miles has been demonstrated. Regarding the 

 lands from which the Grenville sediments were derived, and the 

 floor upon which they were deposited, we know nothing at present. 

 That organisms lived in the waters while Grenville deposition 

 took place those many millions of years ago seems evident from 

 the dissemination of graphite (crystallized carbon) through much 

 of the limestone as well as through certain of the schists and 

 gneisses. 



A glance at the accompanying geologic map will show the very 

 " patchy " distribution of the Grenville rocks, this being due to the 

 fact that the original body of thick strata, which was the universal 

 country rock of the quadrangle, has been badly broken up, lifted 

 or tilted in masses great and small, more or less engulfed and, in 

 some cases, injected or even partially assimilated by the great 

 intrusive bodies of the region. The entire absence of Grenville 

 strata from the anorthosite area is doubtless due to a laccolithic 

 structure of the anorthosite whereby the Grenville was notably 

 lifted or domed over the rising magma, and completely removed 



