THE ADIRONDACK CRAPHITE DEPOSITS 4-1 



It is often possible when the succession of the beds is understood 

 for a given area, to locate the graphite bed by reference to the 

 hanging or the footwall, although the ore itself may not outcrop, as 

 well as to locate important faults and folds. It was found that 

 practically the same rocks, in the same order, occur on the Dixon, 

 Faxon and the Lakeside properties as well as at the Hooper mine. 

 How many different graphitic ore beds there are in the area investi- 

 gated is not absolutely certain, but it seems highly probable that 

 there are at least two distinct beds. 



Since the deposition of the Grenville series and subsequent 

 alteration (metamorphism) they have been penetrated and injected 

 by a series of igneous rocks that welled up from below, cutting 

 them and greatly altering them. The first of these is a white, fine- 

 grained granite strongly squeezed into a gneiss. It is rarely pure, 

 for it absorbed while in the molten condition quantities of the 

 overlying rock. It is almost always highly involved in and with 

 the Grenville quartzites, having frequently soaked through the latter 

 along the original bedding planes, giving rise to " lit-par-lit " 

 injection gneisses. This will be referred to as the Laurentian 

 granite. 1 



Closely related to the granite in age, is a dark igneous rock here 

 called a metagabbro. The significance of this rock and its relation 

 to the Laurentian granite have heretofore been imperfectly known. 2 

 It is difficult to describe the metagabbro so that it can be recognized 

 in the field, but suffice it to say that it varies from a fine-grained, 

 dense, brown-black rock, similar to diabase or trap, to a salt-and- 

 pepper combination, coarse grained and frequently gneissic. That 

 some of it is later than the Laurentian granite has been demon- 

 strated at the Hooper Brothers' and Flake Graphite Company's 

 properties, although Cushing thinks that the greater part of it as 

 shown generally throughout the Adirondack's is older. 3 It was 

 found to cut the Laurentian granite but is cut by the later granites. 

 Furthermore, the Laurentian granite and the metagabbro have been 

 folded with the Grenville series, while the later granites have not. 



1 See H. P. Cushing et al., N. Y. State Mus. Bui. 145, p. 46-47. 177-80, and 

 Bui. 169, p. 21-26; also Am. Jour. Sci., 39:288-94. 



2 The writer belives that this investigation has demonstrated that the Adiron- 

 dack amphibolite is in part (1) sedimentary, a distinct stratum of the Grenville 

 series; (2) in part igneous, this metagabbro; and (3) altered, impure limestones. 

 A careful study of all three types has shown that in the majority of cases it is 

 possible to distinguish them. (See summary of southern area). 



3 H. P. Cushing. Personally communicated. 



