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



Marblehead near the old fort, and on Gerry's Island. In 

 Danvers and Beverly there are numerous small porphy- 

 ritic and pegmatitic masses of this forrnation. At Put- 

 namville, Danvers, foliation in these rocks has produced 

 a form which has received the distinctive name of amphib- 

 olite-gneiss. A technical description of this forrnation 

 will be found in Geological and Mineralogical Notes, Nos. 

 3-4, Bulletin of the Essex Institute, Vol. xxiii. 



No. 5. Quartz-Augite-Diorite. 



This forrnation has three distinct fornas : quartz-augite- 

 diorite, quartz-hornblende-diorite and quartz-augite-mica- 

 diorite. It has its greatest development in Newburyport, 

 Salisbury and Seabrook, and extends in a southwesterly 

 direction through the towns of Georgetown, Boxford, 

 Middleton and Andover. The form quartz-augite-diorite 

 which I have taken as the type is seen in all parts of the 

 area. Thin sections of the rock from Newburyport, at 

 the old quarry opposite Carr's island, and from Salisbury, 

 give the following minerals : uratilized augite with oc- 

 casional masses of typical augite, hornblende, biotite, 

 plagioclase having the extinction angle of labradorite, 

 some orthoclase and quartz, an abundance of chlorite, 

 considerable calcite of secondary origin, numerous crys- 

 tals of apatite, fine acicular crystals of rutile, large mi- 

 cro-zircons, iron-pyrites and magnetite. 



No. 6. Muscovite-Biotite-Granite. 



This is the typical granite of Rosenbusch and only oc- 

 curs in Essex County in the towns of Andover and North 

 Andover. Thin sections show that it contains the follow- 

 ing minerals : orthoclase, microcline, some plagioclase, 

 quartz, muscovite, biotite, hornblende, numerous gar- 

 nets, rutile in long needleshaped crystals, magnetite and 

 limonite. 



