PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 59 



magnetite and apatite are not rare. A quite common feature is 

 the presence of numerous minute black rods which are square or 

 long in section and are probably magnetite. In the specimen 

 from Peach's Neck which was analyzed a reaction between 

 augite and plagioclase has produced small flakes of brown bio- 

 tite along the edges of the latter and extending into its sub- 

 stance. The alkali-feldspar is less automorphic than the plagio- 

 clase, and is often microperthitic. The quartz, which is abun- 

 dant only in the main rock of Fort Sewall, is always interstitial 

 and clear, with minute glass or liquid inclusions. 



The ferromagnesian minerals vary much not only in amount 

 but in kind. A colorless or almost colorless monoclinic pyrox- 

 ene is most abundant. This corresponds in general to diopside, 

 but in certain specimens, Peach's Neck, and in black " schlieren " 

 at Fort Sewall, it has the habit of diallage, a parting parallel to 

 (100) and (oio) being prominent. The diopside shows high 

 extinction angles and carries few inclusions. The diallage, which 

 has a tendency to light brownish hues, is frequently crowded with 

 minute magnetite (?) rods, which in sections parallel to (oio) 

 are arranged parallel with the direction of extinction, at an 

 angle of 34 with the cleavage cracks. They also carry the 

 small brown or opaque plates which are so frequent in the 

 hypersthene of gabbros. These are not pleochroic and are 

 apparently isotropic. In the hyperitic facies from Salem Neck 

 the pyroxene is a light violet augite. The pyroxenes alter 

 easily to uralite, brown hornblende, and biotite. 



Primary hornblende is not abundant and is to be referred to 

 two varieties. In the main rock of Fort Sewall and in speci- 

 mens from Peach's Neck it is pale green or olive-green, not 

 very pleochroic, and automorphic as well as fragmentary. In 

 the basic hyperitic rocks of Salem Neck it is brown, much more 

 highly pleochroic, and is apparently a barkevikite. Biotite, 

 when primary, is greenish yellow or brown, the latter especially 

 in the hyperitic forms. Secondary hornblende and biotite are 

 extremely common, formed usually at the expense of the pyrox- 

 enes, and often in the form of reaction rims. A few crystals of 



