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 (Xo. 222). The exposure on Harvard 

 Street in Somerville is also of this rock (Xo. 210). The diorite forms 

 a facies of the diabase, and includes all the outcrops in the vicinity of 

 the Old Powder House (Xos. 208, 216, 218) except Xo. 210, already 

 mentioned as belonging to the diabase, and the hill north of High 

 Street and east of Highland Avenue (Xo. 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 divei-gent strahlig-kornig 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 

 Xo. 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 (Xo. 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 



