134 
BULLETIN OF THE LABORATORIES 
perhaps the largest dyke encountered, being over 75 feet wide. Its 
contact can be traced with both granite and schist. The former is 
more or less enclosed by branches from the dyke, while the latter is, 
in the main, sharply distinct. Yet in places there is some ex- 
cuse for the confusion of the two. The schist is very tortuous and 
variable and in places is dark, closely granular and might casually be 
mistaken for the diabase. The epidotic character of the schist as well 
as the aphanitic marginal zone of the diabase prevent real obscurity, 
however. 
This diabase is rather coarsely granular and dark in color. The 
microscopic structure is beautifully distinct. Very large elongate 
prisms of brilliantly polarizing labradorite and fresh, pinkish-brown 
augite, the latter perforated in all directions by the former, make up 
the most of each field, though some of the augite is altered to viri- 
dite and epidote (?). The magnetite, of which there is considerable, 
is altered to hematite and limonite. Very little apatite is present. 
(See Plate XII, Fig. 4. ) 
No. nil, from the dyke at the mouth of Dog river, is typical 
of another phase of these dykes. While most of the feldspar gives 
angles indicating labradorite, the paler crystals generally fall below 36°, 
indicating, according to Pumpelly’s method, oligoclase. The augite 
is in long twinned crystals, which are often bent or frayed out at the 
ends. The margin is often greatly altered, (Plate XII, Fig. 8,) pro- 
ducts of alteration being uralite in strongly pleochroic fibrous masses 
suffused with brown, and large flakes of hematite. The numerous 
large grains of magnetite or titanic magnetite altering to titanite, may 
be regarded as autochtones. The augites show very fine interrupted 
fibrous structures, || O, the angle c : c being about 39°. 
No. 1004 is a finer-grained diabase, east of Dog river, contain- 
ing much metallic oxide and numerous greenish grains, with aggregate 
polarization and tendency to be surrounded by iron grains. A -very 
few crystals of orthoclase, with zonary structure (Plate XI, Fig. 4, ) 
are also noticed. 
No. 1019a is a peculiar modification, being a densely black di- 
abase-porphyrite with ilmenite instead of magnetite. The magma can 
be resolved by a high power to an irregularly granular mass, which is 
filled with peculiar grouped rods of the titanic iron. (Plate XIII, 
