34 THE STRATIFIED ROCKS OF ESSEX COUNTY. 



and ti large amount of grains of a slightly greenish tinge, 

 giving, with the polariscope, quite high single refraction 

 and often allowing a rectangular prismatic outline ; parallel 

 extinction commonly giving an aggregate fibrous polariza- 

 tion ; these grains may be andalusite decomposed to mus- 

 covite aggregates. 



On the southeast side of Nahant Head, dipping under 

 the banded limestones, is a typical argellite slate ; micro- 

 scopic examination shows an abundance of muscovite, nu- 

 merous quartz grains with fluid and microlithic inclusions, 

 some of the quartz grains showing the incipient cracks 

 and partings due to crushing, well rounded grains of plag- 

 ioclase probably derived from some gneissic formation, 

 with quartz and numerous microlithic inclusions. The 

 ground-mass is composed of earthy kaolin and fibrous chlo- 

 rite and embedded in it are numerous cubical iron pyrite 

 crystals. This slate is again seen on the north side of lit- 

 tle Nahant where it is interstratified with a coarse mica 

 schist containing much quartz, some of which is of clastic 

 origin and still shows the grains of original quartz sand. 



The nearest bed of metamorphic sedimentary rock is the 

 outcrop near Flying Point, Marblehead Neck. This rock- 

 mass is now a mica schist and is probably a metamorphosed 

 slate ; it is cut and greatly distorted by the eruptive granite. 

 The microscopic structure of this rock-mass as shown by 

 several thin sections that I have prepared is as follows : 

 several grains of microcline well twinned with numerous 

 inclusions of micro-zircons, orthoclase much kaolinized, 

 earthy quartz in angular and rounded grains, some crushed 

 and broken and many showing incipient cracks due no 

 doubt to local metamorphism, much muscovite and bio- 

 tite lying in the plane of the schistosity, a few grains of 

 epidote, fragments of white garnets and numerous large 

 patches of red garnets which are much broken and crushed, 



