230 Collie—Geology of Conanicut Island , JR. I. 
to the northeast, is a ridge one hundred feet high, covered with 
vegetation and nearly buried by drift. A few outcrops are 
found. The specimens taken from the exposures were precisely 
like those of dike no. 4. The rock on the outer portions of the 
ridge is light colored, while within it is schistose in appearance. 
The identity of the two rocks is so complete that they are re¬ 
garded as both of the same origin. This ridge is doubtless a 
dike once continuous with dike number four, but now separated 
from it by faulting. Because of the close resemblance of this 
dike to others of the island no further description is needed. 
The structure of these dikes, mineralogically and petrographi- 
cally resembles that of the minettes , and such is the name ap¬ 
plied to them in the present paper. 
SUMMARY. 
From the evidence presented in the foregoing pages, the 
geology of Conanicut Island may be summarized as follows: 
First, there was a series of slates of unknown age, into which 
was intruded a mass of granite, porphyritic in character. This 
granitic complex became exposed to weathering influences until 
a great bed of debris lay upon its surface. This surface was 
depressed beneath the sea, and upon it was laid a great series 
of clastic rocks which are carboniferous in age, and which have 
been subsequently metamorphosed into schists. The granite 
was, therefore, the carboniferous shore line. 
Following the deposition of the schists there occurred the 
eruption of igneous matter in the form of dikes. Secondary 
action has played a prominent part in the formation of veins, 
notably those of quartz. Also in the replacement of original 
minerals by epidote, calcite, chlorite, etc., the latter mineral 
often occurring in the form known as helminth. 
An unknown amount of erosion has occurred, removing a 
large portion of the several formations. Finally the island was 
covered by a mantle of glacial drift, which is the last recorded 
event in its geological history. 
Beloit , Wis. 
