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 limonite, 

 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 operations 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 

 sandstones indicates that they are transitional forms of theso 

 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. xui, 

 p. 82), and zoisite is of frequent occurrence in the horn- 

 blende epidote gneiss of Essex county. 



June 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 Row- 

 ley. Jeffrey's Ledge is about forty-five miles northeast 



