Geology of Massachusetts. 21 
Medford, Malden, Chelsea and Lynn, on the north of Boston ; and 
in Needham, Milton and Braintree, on the south. This is the oldest 
and most enduring of the porphyries, and, indeed, the hardest of the 
rocks. Its basis is generally compact feldspar, reduced to a homo- 
geneous paste, and of various colors; as light purple, red of various 
shades, brownish black, and greenish gray. ‘The imbedded crystals 
are either feldspar, or quartz, alone, or existing together in the same 
rock : and their colors are very various, though more usually white 
or gray. By these mixtures porphyries are produced, rivalling in 
beauty the best antique porphyry. ‘This rock is polished with so 
great difficulty, that it is rarely used in our country, either for orna- 
mental or useful purposes. But it would be strange if an increase of 
wealth and refinement should not create some demand for so elegant 
and enduring a rock. Whenever this shall happen, the vicinity of 
Boston will furnish every variety that can be desired, and in blocks 
large enough for any purpose. 
The porphyry range on the north of Boston, is most perfect in its 
characters, and in the greatest abundance at any one place; although 
the southern range spreads over a greater extent of surface. In 
Lynn, and some other towns, I have observed blocks of porphyry 
that were brecciated—that is, they were composed of angular frag- 
ments of porphyry reunited. ‘This furnishes a beautiful variety for 
polishing. , 
Sienrtic Porphyry. 
When sienite contains crystals of feldspar imbedded in the mass, it 
is said to be porphyritic; and some varieties of this rock in the eastern 
part of the State are very elegant. Essex County produces some of 
the finest specimens, particularly Cape Aun. Sometimes the imbed- 
ded crystals of feldspar are white, sometimes flesh-colored, and in 
Gloucester, I found a rock in which they were of a very rich bronze 
color. ‘These sienitic porphyries are extremely elegant when polish- 
ed; but lam not aware that they are employed at all for ornamental 
purposes, in this country. 
Porphyritic Greenstone. 
The ingredients of greenstone are often not easily distinguished 
from each other by the naked eye; and when, in such a case, the 
rock contains disseminated crystals of feldspar, it becomes porphy- 
ritic. If these crystals are greenish white, and the base blackish 
