44 THE STRATIFIED ROCKS OF ESSEX COUNTY. 



posed, in which there are numerous inclusions of apatite, 

 epidote and micro-zircon crystals. The ground-mass is a fee- 

 bly polarizing earthy kaolinite in which there are cubes of 

 iron pyrite, masses of chalcopyriteand a little galenite, some 

 zoisite masses are developed in the line of the schistosity 

 of the rock-mass and some of the magnetite is titanic iron 

 around which patches of leucoxene have developed. Judg- 

 ing from the majority of the sections studied it is probable 

 that this gneiss is derived from an igneous rock rather than 

 from detrital material, although some of the sections indi- 

 cate the latter origin. During the mining excitement of 

 1875 and 1876, a boring was made through this gneiss strik- 

 ing limestone at a depth of fifteen hundred feet. Micro- 

 scopic sections of specimens from Black Rock, East Salis- 

 bury, at the mouth of the Merrimac river, show detrital 

 quartz grains and angular fragments, plagioclase much de- 

 composed, numerous plates of biotite arranged parallel to 

 the bedding, some green hornblende, epidote grains, ti- 

 tanic iron and leucoxene. The ground-mass is principally 

 secondary quartz and ferrite, cementing an earthy fibrous 

 kaolinite. With the polariscope the secondary quartz 

 gives the usual wavy extinctions. 



To the south and running parallel to this hornblende ep- 

 idote gneiss is a band of thoroughly crystalline metamorphic 

 gneiss. The finest outcrops are to be found at Middleton, 

 Boxford, Georgetown, Byfield and the Newbury mining 

 region. The microscopic structure of sections of speci- 

 men from Middleton (No. 18) is : quartz grains and patches, 

 plagioclase with numerous inclusions of quartz, biotite 

 and epidote, green hornblende with inclusions of biotite 

 and apatite crystals, some titanite and chlorite ; ground- 

 mass of secondary quartz and ferrite. Microscopic struct- 

 ure of sections from Boxford (Nos. 109, 110) : numerous 

 quartz grains, well rounded plagioclase grains, much ortho- 



