4 BULLETIN OF THE 
have here been studied agree so well with each other, and with Professor 
Wadsworth’s notes on the other localities where the same rock is ex- 
posed, that a complete study of it seems unnecessary to prove its identity 
with the “ diorite.” 
Petrographical Notes. 
Having shown by field observations that the so-called “ diorite” is 
equivalent to the so-called “greenstone,” the rocks will be distinguished 
according to their mineral composition, without regard to coarseness of 
texture. Microscopic examination shows the rock to be either a diabase 
or diorite. The diabase is the principal rock, being found at the quar- 
ries on Granite Street (Nos. 207, 209, 209 a, 214), and at the very ex- 
tensive quarries on Pine Hill (No. 222). The exposure on Harvard 
Street in Somerville is also of this rock (No. 210). The diorite forms 
a facies of the diabase, and includes all the outcrops in the vicinity of 
the Old Powder House (Nos. 208, 216, 218) except No. 210, already 
mentioned as belonging to the diabase, and the hill north of High 
Street and east of Highland Avenue (No. 203). Aside from the 
amphiboloid mineral, the two rocks appear to be almost identical. 
Diabase. — This rock is quite uniform in character, and occupies a large 
part of the area of the dike. A macroscopic examination shows that it 
is composed mainly of a plagioclase feldspar, and an amphiboloid min- 
eral with more or less biotite and pyrite. The proportions of these 
minerals vary considerably, causing the specific gravity to range from 
2.98 to 2.65. In weathering, the amphiboloid mineral is first de- 
composed, causing a complete disintegration of the rock to a coarse 
feldspathic sand. The rock does not show the slightest evidence of lam- 
ination, and the feldspars, which have been little interfered with in 
crystallizing, illustrate well the diwergent strahlig-kérnig arrangement of 
Lossen, the remaining space being largely occupied by hypidiomorphous 
crystals of augite. The feldspar grains are more or less lath-shaped ; of 
a white, pink, or green color; generally striated ; and have an average 
length of about 8 mm. The specific gravity of cleavage pieces from 
No. 222 was found to be 2.638 and 2.643 by determinations with the 
Thoulet solution, though these results are probably much affected by an 
incipient alteration. In a single specimen (No. 214), the cleavage of 
the amphiboloid mineral is so well developed that the mineral can be 
identified as augite in the hand specimen. 
Under the microscope, feldspar and augite are found to be always 
