Petrography of Shcfford Mountain. — Dresser. 205 
Lonstituents 
Essential 
Accessory 
RSSEXYTE 
]'lagioclase 
Orthoclase 
Hornblende 
Augite 
Biotite 
Apatite 
Magnetite 
Sphene 
Socialite 
Nepheline 
NORDMARKYTH 
Microperthite 
Augite 
Hornblende 
Biotite 
Magnetite 
Sphene 
Apatite 
Nepheline 
Sodalite 
PULASKYTE 
Orthoclase 
Plagioclase 
Hornblende 
Augite 
Biotite 
Magnetite 
Sphene 
Apatite 
Sodalite 
Nepheline(?) 
Structure 
Hypidiomorphic Hypidiomorphic Porphyritic 
Trachytic 
Essexyte. Tliis is a rather coarsely crystalline rock of a 
dark gray color and weathering- to a rusty brown. On a fresh 
fracture feldspar is seen to be slightly the most abundant con- 
stituent, and by the aid of the lens part of it can be seen to be 
striated and hence triclinic. A black or dark brown horn- 
blende is the most conspicuous of the dark colored constit- 
uents. This varies considerably in amount and in some of 
the contact phases makes up fully half the rock, but in general 
it is quite subordinate in proportion to the feldspar. In parts 
of the rock considered typical it is also exceeded in quantity 
by a light colored augite, and brown mica is quite abundant. 
A mechanical separation of the rock was made by Mr. O. E. 
LeRoy at the petrographical laboratory of McGill University 
which gave the following results regarding the feldspars — 
when the specific gravity of the Thoulet solution, which was 
employed, was reduced from 2.689 to 2.651 much feldspar 
fell: between 2.651 and 2.562 much also fell, part turbid and 
part clear grains; between 2.583 and 2.524 a smaller amount, 
all turbid. There were no lighter constituents. 
Grains from each of these specimens were mounted and 
ground for microscopic examination. The first was found to 
be rather broadly striated and to have an extinction angle as 
high as 36° measured on the albite twinning lanK^lLx. These 
are accordingly classed as labradorite. The second, in which 
the twinning lamellae are finer, showed only verv low angles 
of extinction in the clear grains, which are referred to oligo- 
clase-andesine. The turbid grains of this weight are found to 
be either zeolitized labradorite or composite grains of ortho- 
