150 GEOLOGICAL AND 



Bulletin of the M. C. Z. (Whole Series, vol. xvi, No. 

 9, Geol. Series, vol. n.) 



In this belt of elaeolite-zircon-syenite there are numer- 

 ous masses of blackish feldspathic rocks two of which are 

 apparently distinct. Several forms of each may readily 

 be seen in the road cuttings and quarries. The first of 

 these two masses, and the one most abundant on Salem 

 neck, is a typical diorite gabbro or, to be more explicit, 

 pegmatitic veins in the diorite, due no doubt to the flow 

 of minerals first crystallized in this diorite magma. As a 

 point for comparison, there is a series of road cuttings in 

 Marblehead through the diorite of the region where these 



O O 



pegmatitic veins are seen in several places. I have pre- 

 pared and studied several thin sections of them from these 

 cuttings which maybe taken as typical of the whole series. 

 They have a microscopic structure as follows : 



No. 1. Jersey St. Augite-diorite : Augite, hornblende, 

 orthoclase, plagioclase, biotite, magnetite, quartz, apatite, 

 micro-zircons and some garnets. The quartz is apparent- 

 ly original as it has inclusions of zircons and apatite. 



No. 2. Abbot St. Augite-diorite : This has more or- 

 thoclase and large masses of apatite crystals in both the 

 orthoclase and plagioclase ; otherwise as in No. 1. 



No. 3. Abbot St. Augite-diallage-diorite-gabbro : 

 Large masses of augite, some diallage, green hornblende, 

 biotite and drusy quartz, masses of large micro-apatite 

 crystals, some zircons and a little apatite. The biotite is 

 of the red color so noticeable in the elseolite-zircon-syen- 

 ite, plagioclase somewhat kaolinized and a little ortho- 

 clase. Some of the augite is seen as inclusions in the 

 hornblende. 



No. 4. Jersey St. Augite-olivine-hypersthene-diorite- 

 gabbro : This rock is perfectly fresh, no decomposition 



