5 NEW YORK STATE MUSEUM 



PRECAMBRIC ROCKS 



The most ancient rocks, which are of Precambric age, occupy 

 about two-fifths of the area of the quadrangle. The term " Pre- 

 cambric '^ is used because these rocks have not, as yet, been corre- 

 lated with either the Archean or the Algonkian. They include both 

 sediments and igneous masses which have been highly metamor- 

 phosed, and represent a portion of the southern border of the large 

 Precambric area of the Adirondacks and underlie all the Paleo- 

 zoic rocks of the quadrangle. 



GRENVILLE SERIES 



The rocks of the Grenville series are the most ancient of the 

 Precambrics and consist of highly metamorphosed sediments. 

 These rocks represent old sandstones and shales together with some 

 limestones which have been so thoroughly crystallized and foliated 

 that most of the original sedimentary characters have been oblite- 

 rated. Within the quadrangle proofs of the sedimentary origin of 

 the Grenville consist in the occurrence of many layers of widely 

 different composition ; of crystalline limestone and quartzite strata ; 

 and of graphite and garnet crystals. While in the field the writer 

 particularly observed the relation of the Grenville to the other rock 

 masses, but not even a suggestion of any older formation could be 

 found. 



Varieties of Grenville. The Grenville, which is so abundantly 

 and magnificently shown and which presents extreme variations in 

 mineralogy, is exhibited under so many different f acies that it would 

 be hopeless to attempt a description of them all. After studying 

 many specimens and microscopic sections, a careful selection of the 

 most characteristic facies has been made and the descriptions of 

 these immediately following will perhaps give the best idea of the 

 Grenville within the quadrangle: 



I Crystalline limestone. This rock, which is medium to coarse 

 grained and calcitic, is sometimes pure and white but it is often 

 mottled with green serpentinous material (ophicalcite) which is prob- 

 ably derived by the decomposition of pyroxene. It is in thin layers 



ipcx). Prosser. Notes on Stratigraphy of Mohawk Valley and Saratoga 

 Counties. N. Y. State Mus. Bui. 34. 



1908. Brigham. Glacial Geologv of Amsterdam, Fonda, Gloversville and 

 Broadalbin quadrangles, N. Y. State Mus. Bui. 121, p. 21-31. 



1910. Ulrich & Gushing. Age and Relation of the Little Falls Dolomite 

 of the Mohawk Valley. N. Y. State Mus. Bui. 140, p. 97-140. 



1911. Miller, W. J. Preglacial Course of the Upper Hudson River. Geol, 

 See. Am. Bui. 22:177-186. 



