22 Geology of Massachusetts. 
green, the rock is the green porphyry of the ancients. In Dorches- 
ter, Brookline, and Roxbury, according to the Messrs. Danas, it oc- 
curs in rounded masses; and in small quantity, in veins, at Marble- 
head. But I have found it in large veins traversing sienite, at Sandy 
Bay, on the north-east side of Cape Ann. Large blocks might be 
thence obtained ; and if polished, it would constitute a truly splendid 
ornament for the interior of a church, or a private dwelling. 
If the feldspar crystals be black, or greyish black, the rock is the 
superb black porphyry of the ancients. This occurs in small beds and 
rolled masses in Charlestown, and in veins of greenstone at Marble- 
head. I found it, also, in a rolled mass, in the west part of Ipswich ; 
the feldspar being of a jet black, and of a high lustre. Polished spe- 
cimens would vie in elegance, to say the least, with the green por- 
The hornblende slate in various parts of the State, but particular- 
ly in the region of the Connecticut river, is frequently porphyritic ; 
and exceedingly resembles porphyritic greenstone ; being, in fact, 
composed of the same ingredients; and differing only in its slaty 
structure, and in the more distinctly crystalline character of the horn- 
blende. The disseminated crystals of feldspar are usually white. In 
Canton and Easton, they are sometimes the compact variety, yet re- 
taining their form perfectly. : 
The magnetic iron ore in Cumberland, (R. I.) is profusely sprink- 
led with crystals of feldspar ; and would doubtless form no mean sub- 
stitute for green or black porphyry. 
Quartz Rock. 
When this rock occurs pure, it can hardly be employed in archi- 
tecture of any kind, on account of its breaking into fragments so ex- 
tremely irregular. But when it takes.a small proportion of mica into 
its composition, it is often divided, with mathematical precision, into 
layers of convenient thickness for building. The best quarry of this 
kind that I know of, is in the west part of Washington, Berkshire 
County, about three miles south-east of Pittsfield village. The lay- 
ers vary in thickness from one or two inches, to one or two feet ; 
thus affording materials for fine flagging stone, as also for walls and 
underpinning. The quantity of this rock at the quarry is very great. 
though quartz rock is usually, of all others, most easily affected 
. by heat, yet that variety from the quarry in Washington, is remarka- 
ble for its power of resisting heat; and it is here employed for the 
