Varieties of Porphy ry. 



443 



bedded in a simple or compound base." " The word porphyry," 

 says a writer in the Edinburgh Review, " signifies at present a rock 

 having a compact basis, through which are scattered crystals of some 

 other minerals." " Since the time of Werner," says the Dictionnarie 

 Classiqe D' Histoire Naturelle, "most mineralogists confin ethe name 

 Porphyry to rocks with a porphyroid structure, composed of a paste 

 of compact feldspar more or less mixed, which envelopes crystals of 

 feldspar ordinarily whitish." Brongniart defines porphyry as hav- 

 ing a " paste of amphibolic petrosilex, red or reddish, enveloping ob- 

 vious crystals of feldspar." 



Since porphyry passes into other rocks, we ought to recollect a 

 remark of Macculloch, that " the term porphyry, when used in geo- 

 logical description, must not always be taken too strictly in its miner- 

 alogical sense." With the latitude which this remark gives, the por- 

 phyry of Massachusetts conforms to the strictest of the above defini- 

 tions. 



Mineralogical Characters. 



h Compact Feldspar. (No. 1206 to 1228.) This mineral, more 

 or less changed by other substances chemically mixed with it, forms, 

 I believe, the basis of all the porphyry in Massachusetts. At any 

 rate, I have found none which I was not able to fuse with a common 

 blowpipe: and this fact, in connection with another that the great 

 mass of our porphyry has a base of well characterised compact felds- 

 par, has satisfied me that this is the predominant ingredient in the 

 whole of it. But this compact feldspar, both that which forms the 

 paste of porphyry, and that which contains few or no feldspar crys- 

 tals, varies exceedingly in color, in toughness, and in the ease with 

 which it can be fused. And to what but to an admixture with more 

 or less of other minerals, can we impute these differences ? 



It seems to me, that in the present state of geological science, we 

 may take it for granted, that compact feldspar has once been melted. 

 But what was the original rock from which it was produced? Judg- 

 ing from the present constitution of the earth's crust, we must suppose 

 that rock to have been one in which feldspar only predominated, but 

 did not exist alone : or in some cases perhaps, the feldspar formed only 

 a small proportion of the whole. The melting down of such rocks 

 would produce just such varieties of compact feldspar as we find to 

 exist. Sometimes they would be almost pure, while at other times 

 they would contain so much silex, or other earth, that they could 



