38 THE STRATIFIED ROCKS OF ESSEX COUNTY. 



almost entirely decomposed, biotite, some nmscovite, and 

 magnetite, ground-mass, a ferruginous earthy kaolin with 

 some fibrous chlorite and a few grains of epidote. 



The quartzite from the same place in thin section shows : 

 quartz grains with numerous fluid inclusions, feldspars much 

 kaolinized and containing numerous inclusions of apatite, 

 tourmaline and epidote, while patches of chlorite are often 

 seen in the line of the bedding ; the whole rock mass is 

 thoroughly saturated with a ferruginous limonite giving it 

 a dirty yellowish color. The microscopic structure of the 

 quartzite from Lynnfield Centre is : quartz grains contain- 

 ing numerous fluid inclusions and incipient cracks, also 

 crushed and broken grains prod need by pressure in the rock- 

 mass during metamorphism, much secondary quartz which, 

 with the polariscope, gives the usual wavy extinctions, some 

 grains of secondary glassy plagioclase, perfectly fresh grains 

 of rnicrocline, orthoclase kaolinized and much decomposed, 

 with numerous inclusions of zircons and apatite crystals, 

 some chlorite and a little biotite. This outcrop is exposed 

 for a distance of one hundred yards and is in some places 

 finely schistose and laminated, while in others it is massive ; 

 the strike is north 20 east, dip 50 west. 



Nearly all of the bed rock of WestNewbury, Groveland, 

 Haverhill, Bradford, Lawrence and Methuen is composed 

 of metamorphic slate. The microscopic structure of a spec- 

 imen of this rock from Ward's Hill, Bradford, section cut 

 across the bedding, is largely of detrital grains and an- 

 gular fragments of quartz, many of which are crushed and 

 broken, orthoclase grains nearly all kaolinized and earthy, 

 biotite mica abundant in the planes of the schistosity, some 

 original muscovite, microscopic zircons, rutile and titanic 

 iron. A section parallel to the bedding shows large masses 

 of chlorite developed as a product of decomposition of the 

 biotite and inclusions, of apatite crystals are abundant. 



