202 
Collie—Geology of Conanicut Island , R. I. 
At the contact between the granite and the slate there is but 
little evidence to show that the granite is eruptive through 
the slate; it is extremely difficult to determine the exact 
relationship between these diverse occurrences. The con¬ 
tact is nearly vertical; the slate abuts directly against the 
granite, the two being firmly welded together. It is always 
possible to obtain hand specimens with both rocks present and 
the contact clearly shown. 
The evidences of the granitic eruption are briefly as follows: 
1st. Fragments of slate occur in the granite, but only in small 
quantities and at rare intervals. 2d. The slate has been baked 
at the contact with the granite. The effect of the heat is no¬ 
ticeable only in a narrow zone a few inches wide. 3d. Apophy¬ 
ses of granite occur in the slate, these are limited in number. 
Veins of pegmatite, which are later in origin than the granite, 
have been frequently mistaken for apophyses. The granite of 
the apophyses in no case resembles that of the parent stock, but 
is much coarser, and contains only limited amounts of mica or 
hornblende. It is more properly an aplite, frequently taking on a 
pegmatitic structure. These evidences of intrusion are confined 
wholly to the contact between the slate and the granite, on the 
east shore. There are no such evidences on the west shore 
where the slate is wanting, and where the schist immediately 
follows the granite. 
The paucity of fragments of slate in the granite at the con¬ 
tact, and the absence of any very marked contact phenomena 
in the slate indicate that the eruption was a feeble one, devoid 
of a portion of its heat, so that its effects were not pronounced. 
That the flow was semi-pasty is indicated by the large and 
abundant porphyritic crystals of orthoclase which occur every¬ 
where in the granitic area. The relationship between the two 
terranes having been briefly indicated, a more detailed descrip¬ 
tion of each will now be given. 
THE SLATE. 
The macroscopic features have already been sufficiently indi¬ 
cated. Microscopically the slate is an aggregation of interlocking 
quartz and feldspar grains, in which sericite, chlorite and epi- 
