ESSEX COUNTY. 265 



Porphyry. 



I Imvc already sufEciently described the only porphyry in the Scrond district. Its porphy- 

 ritic character is obscure, but still there is no doubt of its true nature. It is a compact red- 

 dish feldspar, in which small crystals of the same material are sparsely interspersed. It is 

 columnar, or is disposed to separate in short quadrangular columns, when only slightly 

 weathered. 



I have already spoken of the apparently stratified porphyry near Canon's point, in the 

 road and fields adjacent to the point, and now refer to this locality again, as it is one of the 

 most interesting places for examination in Essex county. As this mass of porphyry extends 

 west, it appears to divide into traji-like veins, which are themselves traversed by the ordinary 

 greenstone. 



From the foregoing remarks, it will be seen that there are few real outbursts of volcanic 

 matter, except the porphyritic mass just described. This spreads over the surface ; while 

 all those rocks termed trap or greenstone are confined to veins of stony matter, which are 

 usually narrow and unimportant. These may have overflowed the channels in which they 

 are now confined, but no such instance has yet been observed. 



Trap has been often taken for iron ore, in consequence of being a heavy stone, and often 

 discolored by decomposed sulphuret of iron. 



Geol. 2d Dist. 34 



