46 THE STRATIFIED ROCKS OF ESSEX COUNTY. 
ter, near the Loaf on Coffin's Beach, occurs a hornblendic 
biotite gneiss. The microscopic structure (specimens Nos. 
88 and 122) : is hornblende with numerous inclusions of 
biotite and quartz, plagioclase grains with inclusions of 
quartz and apatite, magnetite, some patches of limouite, 
epidote, chlorite, and titanite and rutile inclusions in the 
ground-mass which is a fibrous earthy kaolinite. 
In an economic aspect these stratified rocks possess 
special interest, for it is in the rocks of this class in the 
county that the ores of silver, lead, copper, etc., are found 
as shown by the results of the mining operati(Mis in the 
vicinity of Newbury, Georgetown and Boxford. 
The transitional forms of these metamorphic schists and 
gneisses have not been fully studied in the field, but their 
occurrence in connection with the metamorphic slates and 
sandstcmes indicates that they are transitional forms of these 
rocks, the metamorphism being in part due to the great 
pressure and crushing caused by the granite and diorite 
masses which have been erupted through them. And, fur- 
thermore, zoisite, so far as observed, is only found in met- 
amorphic sandstones and gneisses, except as an epigenitic 
constituent of eruptive rocks (Becker, Geology of the Pa- 
cific Slope, United States Geological Survey, Monog. xiii, 
p. 82), and zoisite is of frequent occurrence in the horn- 
blende epidote gneiss of Essex county. 
Jime 3, 1890. 
Note. — On the fishing ground known as Jeffrey's Ledge, 
twenty miles east-northeast from Thatcher's island, at the 
depth of forty-five to fifty fathoms, the fishermen often 
pull up on their anchors and trawl lines, large masses of 
the Olenellus lower Cambrian chert and limestone, iden- 
tical in composition with that at Nahant Head and Kow- 
ley. Jeftrey's Ledge is about forty-five miles northeast 
