74 



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 gradation is not more 

 complete at this place than near many other dikes, but it is 

 better exhibited within a small area. 



The invariable occurrence of the graphite, in portions of the 

 altered belt remotest froQi the dike, 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 condrodite, 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 first 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 epi- 

 doie and other minerals seen in the red shales where these have 

 been baked and altered by the intrusion of dikes of heated trap, 

 the condrodite seems to claim a corresponding origin to that gene- 

 rally attributed to the epidote, which is regarded as derived, in 

 these cases, from the constituents of the rock itself. To trace the 

 source of the condrodite 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 siliceo-magnesian character ; and the well 

 known tendency to the production of specific mineral combina- 

 tions, in a mass whose particles are in a state of at least semi- 



