42 THE STRATIFIED ROCKS OF ESSEX COUNTY. 



of Andover great pond, across West Boxford to Chadvvick's 

 pond and throughout the entire region between Chadwick's 

 and Stiles' pond. Here observation is interrupted by nu- 

 merous drift hills ; but, beyond, on the same strike we find 

 numerous outcrops in Georgetown, on both sides of Grav- 

 elly pond, and in Byfield and the Newbury mining region 

 where there are several good exposures ; also on the Smith 

 mining lands in Newbury where it forms all the bed rock 

 exposed ; it is last seen at Black Rocks, East Salisbury. 

 In some places this gneiss is but a few rods wide while in 

 others it enlarges to nearly a mile in width. 



There are also several areas of this rock to the north and 

 south of this line of strike. One in North Andover near 

 Mr. Lacy's farm on the road to East Boxford covers sev- 

 eral acres, lying between the metamorphic slate on the north 

 and a crystalline gneiss at the south. This formation ex- 

 tends into East Boxford and forms the famous Crooked pond 

 outcrop which is nearly half a mile wide and can be traced 

 by lesser outcrops to Chaplinville, Rowley, a distance of 

 six miles. 



Twenty microscopic sections of the metamorphic horn- 

 blende epidote gneiss taken from every outcrop in the strike 

 from North Reading to Black Rock, Salisbury Beach, give 

 the structure as follows : section across the bedding of spec- 

 imen from the John Jenkins farm, Andover; brown horn- 

 blende allied to green hornblende, magnetite, plagioclase 

 with numerous inclusions of quartz, biotite flakes and 

 masses in the plane of bedding, numerous quartz grains, 

 many of them well rounded and containing numerous fluid 

 inclusions, some patches of chlorite, numerous grains of 

 epidote, a little sahalite and large masses of zoisite. 



Section across the bedding of specimen from east of the 

 Lacy farm, North Andover : green hornblende with epi- 

 dote inclusions, much biotite, quartz grains and patches of 



