34 THE STRATIFIED ROCKS OF ESSEX COUNTY. 
and a large amount of grains of a slightly greenish tinge, 
giving, with the polariscope, quite high single refraction 
and often showing a rectangular prismatic outline ; parallel 
extinction commonly giving an aggregate fibrous polariza- 
tion ; these grains may be andalnsite 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 tibrous 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 wdiich 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 inci})ient cracks due no 
doulit to local metamorphism, much muscovite and bio- 
tite lying in the plane of the schistosity, a kw grains of 
cpidote, fragments of white garnets and numerous large 
patches of red garnets which are much broken and crushed, 
