MINKRALOGICAL NOTES. 151 



being noticc:il)le in any of the minerals. The probable 

 genesis of the crystallization of these minerals from the 

 magma was magnetite, zircon, apatite, augite, olivine, 

 bypersthene, biotite, hornblende, plagioclase, orthoclase 



and quartz. 



The pegmatitic: diorite rock from Salem neck and 

 vicinity in the elseolite-syenite belt has the following 

 microscopic structure when studied in thin sections in 

 polarized light. 



No. 1. Augite-olivine-diorite-gabbro : Numerous well- 

 twinned plagioclase crystals, some orthoclase, green horn- 

 blende, an abundance of perfectly fresh biotite, crystals 

 of olivine, some irregular patches of quartz, and some 

 glassy plagioclases as inclusions in the biotite and horn- 

 blende. Some of the olivine is inclosed in these hornblende 

 masses and is much altered, forming magnetite. Nu- 

 merous lime iron garnets and cubical iron pyrites are also 

 seen as inclusions in the plagioclase. Crystals of apatite 

 and micro-zircons are abundant in all parts of the section. 

 The specific gravity of the plagioclase is 2.69. 



No. 2. Salem neck. Hornblende -augite-olivine-diorite- 

 gabbro : Much green hornblende, good sections of augite, 

 some olivine, large patches of biotite, fine well-twinned 

 plagioclase, some orthoclase, a little quartz, numerous 

 masses of quite large apatite crystals and a few zircons. 

 Some of the olivine is partly altered to magnetite and 

 serpentine. 



No. 3. Salem neck. Hypersthene-augite-olivine-dio- 

 rite-gabbro : Much plagioclase, some orthoclase, bypers- 

 thene, augite, olivine, hornblende, biotite and a little 

 quartz. Otherwise as in No. 2. 



A comparison of the structure and minerals in these thin 

 sections from the Marblehead diorite region with those 

 from the diorite of the elaeolite-syenite region of Salem 



