OF DENISON UNIVERSITY. 
127 
Next, passing up Dog river about one-eighth of a mile from 
its mouth, we encountered a large dyke 15-20 feet in width. No. 1017, 
which in microscopic section presents all the characters of a typical 
diabase similar to the other dark, fine-grained dykes. Its feldspars 
consist of oligoclase with an angle of about 26° and labradorite with 
an angle of about 48°. Some magnetite, otherwise very free from ac- 
cessory ingredients. Adjacent to this dyke is the indurated slate 
forming No. 1018, finely crystalline, containing large quantities of 
brown mica in scales in O plane, also many irregular grains (Plate XII, 
Fig. 5 A), and grains of magnetite broken up into fine powder, also a few 
calcite grains, showing characteristic cleavage. The mica appears to be 
decomposing to chlorite, forming a chlorite and. quartz basis. Pseu- 
.domorph crystals of chlorite after mica are seen (Plate XII, Fig. 5B.) 
Passing farther up the river the slate becomes porpyritic for a consid- 
erable thickness. 
No. 1019 is a sample of this porphyry. It has a very fine quartz 
magma, with brown mica. The orthoclase crystals have more or less 
regular crystalline outlines and its general appearance is strikingly sim- 
ilar to that of the pebbles in the conglomerate No. 1023. In close 
contact with this porphyry is No. 1019a, a dark very fine grained typi- 
cal diabase. Its relation to No. 1019 is shown in Plate XI, Fig. 6. It 
appears from the relations as if the porphyry (No. 1019) was 
once continuous with the conglomerate No. 1023, which is found a 
little farther up the river, but has lost its conglomeritic structure and 
suffered the complete fusion of the pebbles and schist through the in- 
fluence of this dyke No 1019a. 
No. 1023 is the band of conglomerate at lower Denison Falls, 
where it is from 39-75 feet in width. Its ground mass, which is near- 
ly similar to the adjacent schist, is filled with water-worn pebbles of a 
porphyritic rock No. 1023a. (See Plate XII, Fig. 9.) 
No. 1023a. These pebbles have a sharp outline and can be per- 
fectly separated from the 'ground mass. They show unmistakable evi- 
dence of the action of water in rounding their corners and angles be- 
fore their deposition. They also show the jointage planes of the prim" 
itive rocks from which they were derived. Their general external ap- 
pearance is about the same, but under microscopic examination of a 
large number of slides it is clearly shown that they consist of two dis- 
tinct rock species. This distinction is well marked, for there is shown 
no gradation from one to the other, even in pebbles which are only an 
