Wadswortb.] 814 [February 20, 



Numerous dikes occur in this region, some narrow and fine 

 grained, others Avide and more or less porphyritic. The directions 

 taken by these dikes are various (north and south, north 70° east, 

 north 40° west, etc.), following the jointing of the rock. The dikes 

 have sometimes left one fissure in the rock for another, but in gen- 

 eral they follow one of two directions making two sets of dikes, one 

 taking mainly a north and south direction, and the other an east 

 and west course. Their general course is like to that of similar 

 dikes in the vicinity of Boston; and as is the case there, the east 

 and west dikes are the older. The Rockport building granite is 

 composed of grayish white feldspar, colorless quartz, quite abundant, 

 and black mica (lepidomelane, Cooke, annite, Dana), sometimes 

 with cryophyllite and minute crystals of danalite. 



The mica, in the thin section, shows an irregular black mass, blue 

 and dark brown on the thinest parts, accompanied with bundles of 

 brown needle-shaped crystals, and plates that have the microscopic 

 characters of biotite. Whatever the mica may be in the veins, it 

 does not appear to be homogeneous in the rock itself. It occurs in 

 very irregular shapes, in shreds and patches, with fringed and jagged 

 edges. Flakes of the mica are scattered through the feldspar, and to 

 some extent through the quartz. The feldspar is orthoclase, some- 

 what decomposed, full of inclusions, in general characters similar to 

 that of the Quincy syenite, and in similar twins. The quartz is full 

 of fluid inclusions, with their movable and moving bubbles. These 

 inclusions are often arranged in bands and star-like forms. The 

 quartz is more abundant than in the Quincy syenite. 



The hornblende of the Rockport syenite is of shades of blue and 

 green, with spots of reddish brown, and is in the same irregular 

 masses as the mica; the feldspar and quartz have the same charac- 

 ters as that of the granite, but fuller, it may be, of inclusions. In 

 the syenite of Cedar St., Gloucester, the hornblende is of similar 

 character, and the feldspar contains numerous inclusions of horn- 

 blende, similar to those in the feldspar of the Quincy syenite. The 

 quartz, besides its fluid inclusions, contains also many colorless micro- 

 lites. The fine grained granite one-fourth of a mile north of Glou- 

 cester, is composed of quartz, feldspar, and a brown mica of the 

 same microscopic characters as biotite. This mica has in places a 

 bluish shade of color. 



The dike at this locality, running north 60° east, is composed of a 

 fine crystalline, dark greenish gray rock, which in the thin section is 



