METAMORPHIC ROCKS. 
473 
■“Immediately upon the western side of this curious vein, and ranging along the base of the 
hill, occurs the narrow belt of altered limestone. The gradation of change which here exists 
between 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 subjected to the igneous agency, is its passage from 
the ordinary earthy texture to a subcrystalline 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 development of 
the graphite, as yet seen only in small but very brilliant scales, which are oftentimes hexa¬ 
gonal. 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 com¬ 
pletely has the injected matter of the vein been mingled, in many places, with the fused sub¬ 
stance of the limestone, that no distinct line of demarcation is discernible between them. 
“ The series of changes here described may be considered as representing the phenomena 
in every instance, where superficial deposits have not concealed the vein, the blue limestone, 
and the intermediate altered belt. The locality above referred to is dwelt on in detail, chiefly 
because it furnishes a distinct exhibition of each successive stage of the change. The grada¬ 
tion is not more complete at this place than near many other dykes, but it is better exhibited 
within a small area. 
“ The invariable occurrence of the graphite in portions of the altered belt remotest from 
the dyke, and its never existing in more than a very trivial quantity, even adjacent to the 
vein where the other extraneous minerals are frequently present in great excess, strongly imply 
that it has been derived from the elements of the blue limestone itself, which may easily be 
proved to contain an adequate quantity of iron and carbon for the production of this mineral. 
“ It is not a little curious that, in some belts, the altered rock contains the mineral chon- 
drodite, in a precisely similar relation as to the degree of crystallization of the mass, and 
proximity to the vein of igneous matter; that is to say, when it fir.st appears, in the portions 
of the crystalline belt remotest from the line of injected minerals, it is in small imperfectly 
developed nuclei, which grow larger and better formed as we approach the quarter of more 
intense igneous action, but which, like the crystals of graphite, usually remain but sparsely 
disseminated through the rock. Showing a strong analogy, in its mode of distribution through 
the substance of the white limestone, to the nuclei or geodes of epidote and other minerals 
seen in the red shales, where these have been baked and altered by the intrusion of dykes of 
heated trap, the chondrodite seems to claim a corresponding origin to that generally attri¬ 
buted to the epidote, which is regarded as derived, in these cases, from the constituents of 
the rock itself. To trace the source of the chondrodite upon this hypothesis, we have only 
to conceive that the injected mineral matter, in an igneous state, was poured through fissures 
in a limestone, possessing, what is very common, a silico-magnesian character; and the 
well known tendency to the production of specific mineral combinations, in a mass whose 
Geol. 1st Dist. 60 
