30 The American Geoluyist. Jnly, 1895 
closely associated with the intrusive mass represented by 
Owl's head. 
A microscopic examination of the dark colored rocks re- 
veals some interesting petrographical relations. As was an- 
ticipated, the examination of the series ])roved them to be 
made up of the same mineral mixtures as were found along 
the shores of lake Champlain (Bull. 107, U. S. G. S.). The 
intrusives, therefore, as found along the shores of lake Mem- 
phremagog may be enumerated as follows: Olicine diabase, 
camptonite, auf/ite ca))ijitonile,/ourcIiife and monchiquite. 
Micuoscopic Description of Dikes. 
Granite Dikes. 
This variety predominates along the eastern shore of the 
lake immediately north of the mouth of Fitch bay. A few 
narrow dikes were also found intersecting an island opposite 
Owl's head. 
Thin sections revealed well developed phenocrysts of feld- 
spar, as well as exceedingly minute lath shaped crystals, the 
latter making up at least eighty per cent, of the rock and 
constituting the ground mass. In some cases the ground mass 
has suffered mechanical deformation, thus inducing an appar- 
ent clastic structure. When the clastic magma prevails the 
feldspar phenocrysts become comparativel}^ rare. When the 
micro-crystalline structure predominates the porphyritic feld- 
spars become prominent and generally show twinning struc- 
ture. 
Quartz occurs as large irregular grains showing little or no 
crystalline form, and as fine grains making up a portion of 
the ground-mass. The larger individuals ccuitain numerous 
streaks and clouds of fine dust, probably magnetite, as well 
as minute hair-like crystals with high index of refraction. 
Cross sections show hexagonal outlines. The cr^'stalline 
form and high refraction suggest that the inclusions may be 
apatite. Under a high power lens numerous globules and 
acicular aggregates were also detected. 
Mica is the only remaining constituent of any importance. 
Biotite is by far the most prominent, but a few flecks of mus- 
covite have been detected in one or two sections. The biotite 
occurs as large irregular plates or bunches. The larger flecks 
are invariably frayed out on the edges as if they had been 
