REPORT ON THE GEOLOGY OF ESSEX CO., MASS. 123 



tact it is a coarse, friable, reddish-colored rock ; at the 

 centre of some of the larger masses it is fine grained and 

 compact, resembling quartz porphyry, while it is often 

 seen, in portions of larger masses, with a distinctly strati- 

 fied form. This last form is clearly due to flows or 

 streams of the minerals in the magma of which the rock 

 was composed previous to its consolidation. Near Bass 

 Rock, East Gloucester, every variety of this rock-mass 

 can be seen near the contacts of the augite-syenite and the 

 hornblende-granitite. Microscopic investigation of thin 

 sections of this rock shows that it is composed of the 

 following minerals : quartz, orthoclase, augite, hornblende, 

 biotite, colorless garnet, magnetite, iron-pyrite, glauco- 

 phane. The orthoclase has intergrowths of albite forming 

 microperthite and these microperthite areas are honey- 

 combed with quartz forming a micropegmatitic mass which, 

 under cross nicols in polarized light, resembles a beautiful 

 mosaic. 



No. 3. Augite-Nepheline-Syenite. 



This number Covers a great variety of forms of this 

 rock-mass which on a map of a greater scale could be 

 subdivided as elaeolite-zircon-syenite, mica-syenite, horn- 

 blende-syenite, quartz-hornblende-syenite, quartz-augite- 

 syenite and segerine-syenite ; besides pegmatitic masses 

 containing sodalite, segerine, acmite, ainigmitite, hy- 

 dronephelinite, zircons and lepidomelane. There are 

 numerous areas in which foliation is developed as in No. 

 2. For a f ulier description of these rocks see Geological 

 and Mineralogical Notes, No. 6, Bulletin of the Essex 

 Institute, Vol. xxv. 



No. 4. Hornblende-Diorite. 



In part of this area the rock-mass is distinctly an augite- 

 hornblende-diorite. This is particularly well seen at 



