148 Review of the New York Geological Reports. 



This test of the origin and geological position of a limestone, 

 by the presence of graphite, does not seem to be so conclusive in 

 all parts of the United States as it would seem to be in New York. 

 Professor H. D. Rogers, in his report on New Jersey, describes a 

 locality, four miles southwest of Sparta, in that state, where there 

 is a belt of limestone containing graphite, which he considers as 

 belonging to formation II, i. e. the equivalent of the Black River 

 and Trenton limestones of New York, but altered by the vicin- 

 ity of a granitic dyke. Speaking of this locality on p. 73 of the 

 New Jersey report, Prof. Rogers remarks : — 



"Immediately upon the western side of this curious vein, and 

 ranging along the base of the hill, occurs the narrow belt of al- 

 tered limestone. The gradation of change which here exists be- 

 tween the blue and earthy limestone, and the white crystalline 

 rhombic spar, is distinctly traceable as we approach the igneous 

 dyke. In a breadth not exceeding fifty feet, we discover every 

 degree of modification which the rock can undergo by heat. 

 The first intimation which the limestone gives us of its having 

 been subject to the igneous agency, is its passage from the ordi- 

 nary earthy texture to a sub-crystalline one. 



" We next behold a slight change of color to a lighter tint 

 of blue, and at this stage of alteration, we notice the first devel- 

 opment of the graphite, as yet seen only in small, but very bril- 

 liant scales, which are oftentimes hexagonal. Very soon the 

 mass becomes mottled with white, minutely granular carbonate 

 of lime, the spangles of graphite growing progressively larger. 

 Approaching still nearer to the dyke, the whole rock assumes 

 the white sparry character, and contains near the line of contact, 

 besides the graphite, several of the numerous crystalline minerals 

 of the vein itself. So completely has the injected matter of the 

 vein been mingled, in many places with the fused substance of 

 the limestone, that no distinct line of demarcation is discernible 

 between them." And on p. 74 he goes on to say : " The inva- 

 riable occurrence of graphite in portions of the altered belt re- 

 motest from the dyke, and its never existing in more than a very 

 trivial quantity, even adjacent to the vein where the other extra- 

 neous minerals are frequently present in great excess, strongly 

 imply that it has been derived from the elements of the blue lime- 

 stone itself, which may easily be proved to contain an adequate 

 quantity of iron and carbon for the production of this mineral." 



