IGNEOUS ORIGIN OF LIMESTONE. 43 



" the igneous dyke. In a breadth not exceeding fifty feet, we discover every degree of modi- 

 " fication which the rock can undergo by heat. The first intimation which the hmestone 

 " gives us of its having been subjected to igneous agency, is its passage from ordinary 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 the alteration, we notice the first developcment of the graphite, 

 " as yet seen only in small but veiy brilliant scales, which are often times hexagonal. Very 

 " soon the mass becomes mottled with white, minutely granular carbonate of lime ; the span- 

 " cries of graphite growing progressively larger. Approaching still nearer to the dykes, the 

 " whole rock assumes the white sparry character, and contains, near the line of contact, be- 

 " sides the graphite, several of the numerous crystalhne minerals of the vein itself. So com- 

 " pletely has the injected matter of the vein been mingled, in many places, with the fused 

 " substance of the limestone, tiiat no distinct line of demarcation is disccrnable between 

 " them."* 



The above extract contains all ihat is important in relation to the alteration which the lime- 

 stones are supposed to have undergone by proximity to the igneous rocks. In relation to the 

 passage, I would remark, that the inferences must be true, if the premises from which they 

 are drawn are correct ; and I should not be so much disposed to question them, if they were 

 confined to the single locality from which Prof. Rodge^p has drawn them ; but inasmuch as 

 these veins apply to several ranges of limestone, comprehending numerous bands, and extend- 

 ing in belts from beyond Amity in Orange county in New-York, southwest to Andover forge, 

 the question assumes a different aspect ; for having examined what are termed the altered 

 belts of limestone, at Amity and several other localities in that region, my own conclusions 

 are unfavorable to the metamorphic theory when applied to tlie limestone in question. It is 

 entirely unessential to state what an observer did not see in this case, for that would be no 

 proof positive that certain facts and phenomena did not exist ; but I did see wliole beds of 

 limestone lying between primary rocks, charged with graphite, condrodite, pyroxene, spinelle, 

 etc. all varying much in the perfection of the crystallization. 



In relation to the whole matter, so far as the counties of Orange and Sussex are concerned, 

 I am free to confess that my opinions rest partly upon analogy. I find in St. Lawrence, 

 Essex and Jefferson counties, beds of limestone so situated as to preclude wholly the infe- 

 rences of Professors Rodgers, Hitchcock and others. Mineralogically it is the same lime- 

 stone ; it contains spinelle, condrodite, pyroxene, graphite, hornblende, mica, scapohte, etc. ; 

 making the analogy between the limestone of the two sections of country complete. Be- 

 sides, it appears that the reasoning of Prof. Rodgers proves too much. Tlie extent of the 

 influences of this dyke of granite, in the instance which is so well detailed, is greater and 

 more extensive than the nature of the case will warrant. Melted rock, sufficiently fluid to 

 flow, is chilled by contact with other rocks ; the cooled surface then becomes an imperfect 

 conductor of heat; andthough the mass in the interior remains fluid for a long time, still 



* Prof. H. D. RoDOEKs's Fuial Report of the Geology of Neio-Jersey, p. 72, 73. 



