REPORT OF THE DIRECTOR I914 4.-^ 



prominence which are capped by the harder siHcate rocks, while 

 the larger valleys are floored by limestones and associated schists. 



The main elements in the rock series are gneisses. These show 

 considerable variations in mineralogical composition and appear- 

 ance from place to place but, broadly viewed, are quite uniform as 

 they consist of certain fairly well-defined types which recur 

 throughout the area. There are dark hornblende gneisses in which 

 the hornblende shares importance with the feldspathic minerals and 

 which are further characterized by the common appearance of 

 epidote as a secondary alteration product. As a rule they are of no 

 great extent in any one section, but reappear frequently in belts or 

 bands within the other gneisses. A lighter variety of gneiss carries 

 hornblende or biotite in subordinate amount, consisting mainly of 

 feldspar and quartz in the proportions found in granites. The 

 feldspar consists of both alkali and calcic varieties. The texture is 

 usually fine granular, revealing little as to the antecedents of the 

 gneiss. The commonest type is banded biotite or hornblende gneiss 

 in which the dark components are segregated in parallel bands with 

 lighter material between. The bands are usually thin and quite 

 regular, lending the appearance of a bedded arrangement. This 

 type resembles the characteristic Fordham gneiss of Westchester 

 county which, as suggested by Berkey, probably continues into the 

 Highlands without interruption. All these rocks contain much 

 igneous material in the form of dikes, stringers and other intruded 

 bodies, or that has been absorbed into the mass in disseminated con- 

 dition, so that often their aspect is quite as much conditioned by 

 this added material as by the original constituents. 



The granitic rocks predominate among the igneous class and are 

 made up of ordinary granites with mica or hornblende as dark 

 minerals, of fine-grained aplitic phases, and of coarser textured 

 varieties classed as pegmatite. The normal granites form bosses 

 and stocks of considerable extent and probably of several different 

 periods of intrusion. The latter inference is based on the varied 

 degrees of regional metarnorphism which they exhibit ; some are 

 practically unchanged from the condition of their first consolidation 

 and others show granulation, recrystallization, or development of 

 characteristic gneissic appearance. It would seem therefore that at 

 least two periods of intrusion are represented and very likely more. 

 Pegmatite is abundant in the vicinity of most of the mines, and in 

 some cases is closely related to the ore occurrence. It occurs in 

 dikes and lenticular masses, often in thin veins interleaving the 

 gneisses in the same way as the finer grained granite. Its almost 



