304 



TRANSACTIONS OF THE 



This rock appears to have been once in a state of semi- 

 fusion, and the cavities formed at that period have been 

 partially filled by the reunion of petro-silicious matter y 

 sometimes the crystallization appears to be the effect of 

 coeval sublimation or interior infiltration. Whatever 

 may have been the mode of formation of these rocks, like 

 other variolites, and argilo-porphyries, they appeared to 

 be expressions of pre-existing rocks^ modified by some 

 cause not now acting. 



PORPHYRY. 



This rock appears to be a modification of the preced- 

 ing; a white petro-silicious or feldspathic substance, par- 

 tially crystallized, is imbedded in a compact hard homo- 

 geneous base of a hvown colour. It resembles a dioritic 

 porphyroid, rather than a perfect porphyry. The white 

 imbedded substance exists in spheroids, with a structure 

 semi-crystalline, and not in prisms. The embedded and 

 embedding substances are about equal in quantity. The 

 ovoids are from the size of a pea, down. The base of 

 the rock has been partially penetrated by the substance 

 forming the spheroids. In the preceding rock the ovoids 

 sometime separate from the base, leaving cavities and 

 protuberances. This mineral breaks with an even frac- 

 ture, and the line of separation divides the spheroids and 

 the base^ without any derangement of direction. Both 

 parts appeared to be of equal hardness, and susceptible 

 of a fine and equal polish. The colour of the base and 

 of the contained masses being so distinct, the eifect pro- 

 duced by a polish is very agreeable. 



TRAPPITES — EPIDOTIC ROCKS. 



The summit of the Blue Ridge descending one mile to 

 Paris, is chiefly composed of a green rock which varies 

 much both in colour and composition. It is frequently 

 a light leek green, or a dark green passing into an almost 



