Varieties of Porphyry. 



445 



so called of Saugus ; and probably it is essentially the same thing, 

 viz. compact feldspar with a mixture of some other ingredients. Both 

 of them, are fusible with some difficulty into a semi-transparent po- 

 rous glass. They correspond pretty nearly in their characters to the 

 rose petrosilex of Sahlberg, described by Berthier in the 36th volume 

 of the Annales de Chimie et de Physique, and which he regards as a 

 distinct species from compact feldspar, and which Beudant has since 

 described as such. However, it seems to me that if we make a dis- 

 tinct species of this variety, we must make a dozen others from the 

 compact feldspar of Massachusetts. He would erect this into a new 

 species, chiefly because it differs from feldspar so much in composition. 

 But if compact feldspar had an igneous origin, should we not expect 

 its composition to vary very much, according to the greater or less 

 quantity of foreign substances that happened to melt and mix with 

 the feldspar : nor would this be a reason for making distinct species, 

 so long as the constituents of feldspar predominated. Some speci- 

 mens of our compact feldspar of a reddish color exhibit traces of a 

 schistose structure, and are even traversed by thin veins and layers of 

 quartz. These melt with great difficulty. 



These are the principal varieties of color that I have met with in 

 the non-porphyritic compact feldspar of Massachusetts. When por- 

 phyritic, however, it exhibits several other predominant colors, which 

 will be mentioned in the proper place. 



2. Antique Porphyry. (Porphyre antique, Al. Brongniart.) This 

 variety constitutes the great mass of the porphyry in the vicinity of 

 Boston : and I call it antique because it so closely resembles that used 

 in the monuments and ornamental furniture of the ancients. As to 

 this point we have the testimony of Prof. A. Brongniart, who quotes 

 " Chelsea near Boston;" as a locality of porphyry, and says that " it 

 entirely resembles the antique porphyry."* He might have added 

 that probably as many, if not the same varieties occur in the vicinity 

 of Boston, as were employed by the ancients. The specimens which 

 I have placed in the collection, and the most of which are polished, 

 will render this statement probable. For if 1 could obtain so many 

 varieties during the little time that I have spent in examining this 

 formation, how extensive a suite might be brought to light by long 

 and careful research ! 



The compact feldspar that forms the base of these porhpyries pre- 



* Classification des Roches, p. 108. 



