THE PETROGRAPHICAL PROVINCE OF ESSEX COUNTY. 795 
an evenly granular, granitic structure, varying in texture from 
moderately to very fine grained and compact. In color they 
vary correspondingly from light to very dark gray the most 
compact forms being the darkest. To the naked eye they show 
the same minerals as the granite, but the ferromagnesian con- 
stituents are more abundant. The contact with the surrounding 
granite is nearly always sharp and clear. 
Under the microscope the same essential minerals are seen 
as in the granite and their habit is much the same, except that 
all is ona smaller scale. The alkali feldspars are quite similar, 
hornblende and biotite occur in all cases, the former being 
generally more abundant. It is usually green and only occa- 
sionally shows a bluish hue. In certain rather dark masses from 
the Pigeon Hill quarry, near Rockport, there is found a peculiar 
hornblende which occurs in bundles of long slender rods. In 
color it varies from colorless to pale bluish-gray, the latter 
shows brilliant polarization colors especially on the edges, and 
extinguishes obliquely, ¢/\¢c being about 15°. The pleochroism 
is rather strong, | ¢ colorless or pale yellow, II ¢ bluish or deeper 
yellow-gray, though occasionally it shows no pleochroism. It 
The biotite is also like 
It is sometimes secondary about horn- 
is apparently a variety of glaucophane. 
that of the granites. 
blende, and in one specimen from Bay View it occurs about 
Colorless diopside 
is not abundant and often shows signs of alteration. 
magnetite in brown flakes arranged radially. 
Mag- 
netite, while not abundant, is yet more common than in the 
granite. 
Hill quarry forms well shaped but corroded crystals of con- 
Zircon is common and in specimens from Pigeon 
siderable size, one being 1.5"™ long. Apatite is present as an 
accessory, but allanite was not observed. 
SiO, - - = 7/36 MgO - =) 1) (0:03 
TiO sy e= - 0.60 CaO) = - 0.55 
Al,O3 - - 15.05 Na,O . = 4.42 
Fe,O3 - - T23 K,0 - - 6.08 
FeO - - - 4.76 H,O (110°—) 0.16 
MnO =- - 0.05 H.O (110°+) 0.17 
100.45 
