2o6 The American Geologist. October looi. 
clase and a heavier constituent. The H,c:htest constituent was 
found to be unstriated feldspar presumably all orthoclase. It 
fell in smaller amount than either of the others. 
In the thin section of the rock the labradorite is found in 
better formed crystals, but in smaller amount than the olig^o- 
clase-andesine, while orthoclase, which can often be distin- 
guished by its incipient alteration, is the least abundant of the 
three feldspars. 
Aug^ite is colorless and without pleochroism. It shows 
characteristic cleavage, nearly right angled, and extinction 
angles as high as 45°. 
Hornblende is brown varying in shade from deep chest- 
nut to yellowish. It is trichroic, the scheme of absorption be- 
ing c > t > a. The greatest angle of extinction that was 
observed c A c was 27°. 
Biotite in some instances is intergrown with horneblende, 
and in others encloses numbers of augite crystals promis- 
cuously arranged, thus giving an excellent micro-poikilitic 
structure. 
The structure of the rock is hypidiomorphic and the order 
of crystallization, the normal one for plutonic rocks. The 
ferromagnesian minerals enclose the basic accessories and are 
themselves generally of an earlier order of crystallization than 
the feldspars. Of the latter the labradorite has crystallized 
earlier than the oligoclase-andesine and both are cemented 
together by orthoclase. 
The rock thus agrees essentially with the characters of 
the essexyte group, a near analogue probably being the augite 
dioryte of Rosita Hills, Colorado,* described by Cross (XVII, 
An. Rep. U. S. G. S). A specimen of essexyte from Salem 
Neck, Massachusetts, kindly given me by Mr. J. H. Sears of 
the Salem Academy of Sciences, agrees completely with some 
of the darker and more hornblendic portions of the Shefford 
mass. 
The following are the results of (I) analysis of a specimen from 
Morriseau's quarry, Shefford, which, judging from the microscopic evi- 
dence, is probably the most acidic part of the intrusion;t (II) Augite- 
dioryte, Rosita Hills, Colorado; (III) Essexyte, Rangstock, Bohemia 
*This has been included by Prof. Rosenbusch in the essexyte group. — "Ele- 
mente Gesteinslehre." 
i-A. few small grains of quartz were found in a section from this specimen, 
the only quartz tiiat has been found in this rock. 
