44 THE STRATIFIED ROCKS OF ESSEX COUNTY. 
posed, in which there are immeroiis inclusions of apatite, 
epidote and micro-zircon crystals. The ground-mass isa fee- 
bly polarizing earthy kaolinite in which there are cul)es 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 dctrital material, although some of the sections indi- 
cate the latter orioin. 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 fcriite. Micr()scoi)ic sti'uct- 
ure of sections from Boxford (Nos. 109, 110) : numerous 
quartz grains, well rounded plagioclase grains, nuich ortho- 
