THE GARNET DEPOSITS OF WARREN COUNTY, 



NEW YORK 



BY WILLIAM J. MILLER 



INTRODUCTION 

 The principal garnet mines of the United States are located in 

 Warren and Essex counties of the eastern Adirondacks, those of 

 Warren county — especially the Hooper and Rogers mines below 

 described — being the greatest producers. All the Warren 

 county mines are in its northwestern portion and within six or 

 eight miles of North Creek village which is at the terminus of 

 the Adirondack branch of the Delaware and Hudson Railroad. 



GENERAL GEOLOGIC FEATURES 

 The garnet mines of Warren county lie wholly within the pre- 

 cambrian rock area of the Adirondacks. The oldest rocks in the 

 garnet region are the highly metamorphosed sediments of the 

 Grenville series. Detailed mapping by the writer has shown ex- 

 tensive areas of Grenville which are unusually rich in limestone 

 and closely associated hornblende gneiss. 



Next in age come plutonic igneous rocks such as syenite, gran- 

 ite, and granite porphyry which are clearly intrusions into the 

 Grenville and all of which are differentiation products from the 

 same great cooling magma. Of these rocks the syenite is, per- 

 haps, the most abundant. It is a medium to fairly coarse 

 grained, generally quartzose and hornblendic rock with some- 

 times a more basic variety carrying a green pyroxene. The gran- 

 ite is highly quartzose and always contains hornblende or biotite 

 or both. The granite porphyry is biotitic to sometimes horn- 

 blendic with large, pink, feldspar crystals imbedded in a fine to 

 medium grained matrix. All these rocks are distinctly gneissoid. 

 As a result of the great intrusion, the Grenville in some cases 

 appears to have been pushed upward and to have been largely 

 removed by erosion since ; in other cases the Grenville was more 

 or less engulfed by, or involved with, the molten flood as shown 

 by the numerous inclusions and the areas of mixed gneisses ; 

 while in still other cases the Grenville rocks were left practically 

 intact as shown by the large areas of pure Grenville. 



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