THE STRATIFIED ROCKS OF ESSEX COUNTY. 41 



from the last-named outcrop is composed of quartz grains 

 and angular fragments 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, inters'trat- 

 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 rwell rounded original grains of 

 quartz and plagioclase, 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. BULLETIN, VOL. XXII 3* 



