1878.] 315 [Wadsworth. 



seen to be composed of plagioclase, light brown augite, magnetite, 

 and viridite or chlorite. Its structure is basaltic, and it is evidently 

 an altered basalt, but has now the mineral composition of a diabase. 

 The dike at the same locality running north 10° east, is a dark gray 

 compact melaphyr, showing in the thin section that it is composed of 

 augite, plagioclase, magnetite and viridite, with traces of olivine. A 

 single crystal of hornblende was seen in the section. This rock has 

 a basaltic structure, and more the characters of a melaphyr than 

 those of a diabase. The rock of the dike in this locality, which 

 runs north 10° west, is of a dark gray color, with grayish white 

 streaks running through it, arising from the decomposition of its 

 feldspar. It is finely crystalline, and in the thin section shows great 

 alteration. It is composed of feldspar with opacite, viridite, and 

 other products of alteration. 



Thin sections were made of material obtained both from the cen- 

 tre and the. edge of one of the large north and south dikes, in the 

 principal quarry of the Rockport Granite Company. The exterior 

 of the dike is a compact grayish black rock, finely crystalline, and 

 made up of augite, plagioclase, magnetite, and viridite; while the 

 centre of the same dike is composed of a dark gray ground mass, 

 porphyritically inclosing crystals of plagioclase, well striated. 



In the thin section it is seen to be composed of plagioclase, magne- 

 tite, opacite, and a green fibrous product of decomposition, somewhat 

 dichroic, and doubtless of hornblendic character. In blocks of the 

 syenite, used in a wall at the side of the railroad track, about a quar- 

 ter of a mile south of the Rockport depot, narrow dikes are seen, 

 varying from a few inches to less than an eighth of an inch in width. 

 A thin section was made, showing the syenite on both sides of a dike 

 about a third of an inch wide. The dike is composed of a brown 

 tachylitic glass, showing flow structure, and curving around the few 

 inclosed crystals, which are mainly feldspar and magnetite. The 

 quartz of the adjoining syenite is full of colorless microlites. 



The examination of the thin sections, so far as it has been carried, 

 would not show any essential difference between the syenite of 

 Quincy and that of Rockport, but would rather indicate that they 

 are of the same age, as has been supposed heretofore. The syenite, 

 in the thin section, has a similar character and structure to that of 

 the granite, so much so, that in some cases, especially in the thicker 

 sections, it is quite difficult to distinguish between them. The mi- 

 croscopic study, therefore, strengthens the conclusions drawn from 



