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regularly as we approach the dike. The crystalline nuclei, first 

 extremely minute and faintly marked, become progressively 

 larger and better defined, assuming when within a certain, 

 though still considerable distance, all the characters of a fully 

 developed mineral, possessing in some instances the structure 

 of a perfectly regular isolated crystal. As this gradual segrega- 

 tion of certain specific mineral combinations increases, we notice 

 a progressive obliteration of all the ordinary features of the 

 rock. 



Another highly interesting point is the intimate connexion, che- 

 mically, in every instance, between the composition of the stra- 

 tum acted on, and the sprxies of mineral secreted. Thus in all 

 the principal localities of the epidote, we find the red shale con- 

 taining by analysis all the constituents essential to that mineral, 

 even the lime. Moreover, the red sandstone, so usually destitute of 

 lime, is singularly enough the stratum in which the tourmaline, a 

 mineral not holding any lime, abounds. 



Where the rock has been a calcareous and very argillaceous 

 red shale, the mineral produced has invariably been epidote, the 

 ingredients of which are silica, alumina, lime, oxide of iron, and 

 a little manganese, the five prevailing constituents of the red shale. 

 On the other hand, where the rock has been originally a some- 

 what more arenaceous compound, such as usually goes by the 

 name of argillaceous red sandstone, a triple mixture of alumina, 

 silica, and peroxide of iron, there we find, in place of epidote, 

 a profusion of black tourmalines, the composition of which, 

 as analyzed by Kloproth, is 40 alumina, 35 silica, and 22 oxide 

 of iron. 



There are several other imperfectly crystalline minerals some- 

 times associated with the above, but I have not yet found time 

 to investigate their composition and probable origin. 



A careful examination of the altered rocks near the trap would, 

 I doubt not, result in much useful information of a strictly prac- 

 tical kind, in reference especially to a right selection of stone for 

 purposes of architecture. I conceive that the very best varieties 

 of building rock in this region of the State, will be found in the 

 indurated strata, contiguous to the ridges of trap, where the baking 

 process has obliterated the slaty cleavage, and destroyed that 

 soft friable texture which usually renders so large a portion of 



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