THE STRATIFIED ROCKS OF ESSEX COUNTY. 41 
from the last-named outcrop is composed of quartz grains 
and angular frajrments with numerous fluid inclusions show- 
ing incipient cracks and broken grains, much muscovite, 
some biotite, magnetite, chlorite and epidote. 
Another large area of the metamorphic slate, interstrat- 
ified with sandstone, first seen near a small pond in South 
Groveland, is nearly continuous from Johnson's pond in 
West Boxford to the north side of Chadwick's pond in 
Bradford and forms all the adjoining outcrops for nearly 
two miles in North Andover. The microscopic structure 
shown by several sections is :well rounded original grains of 
quartz and plngioclase, biotite, muscovite, a little chlorite 
cemented by a thin film of secondary quartz and ferreous 
oxide. One of the sections contains magnetite and limo- 
nite. The sandstone is composed of nearly pure quartz 
sand cemented by some secondary quartz and a fibrous fee- 
bly polarizing felspathic mass ; fluid inclusions in which the 
bubble movement is quite active are frequent in the quartz 
grains. In a cutting of the Boston and Maine Railroad just 
north of Reading, Middlesex county, is a fine exposure 
of hornblendic gneiss showing great variety. Succeed- 
ing this on the west are sandstones, diorites, coarse gran- 
itic gneisses and eruptive granite dykes ; on the east are 
the eruptive hornblendic granites and crystalline gneisses 
of Middleton. The strike of the whole series is north 40*^ 
east, dip varying from 30° north of west to 90°. 
Following the strike of the hornblendic gneisses into 
Essex county there are outcrops in various places near Fos- 
ter's pond, Andover, and on the roadside, in a cutting near 
the John Jenkins farm, there is an exceptionally good ex- 
posure where this gneiss is seen for several rods with the 
same strike with the dip slightly to the west. Continuing, 
numerous exposures are seen in Farnamville, North Ando- 
ver, at Marble Ridge near the railway station, on both sides 
ESSEX INST. BULLKTIN, VOL. XXII 3* 
