208 
Collie—Geology of Conanicut Island, R. 1. 
mass are micro-granitic, but usually it is coarse in character. 
The granite possessing a porphyritic facies, resembles granite 
porphyry, its mode of occurrence, however, is intrusive. 
Zircon, apatite and the titaniferous minerals belong to the 
earliest stages of crystal development, the micas follow, quartz 
and feldspar forming last. The feldspars precede the quartz, the 
plagioclase always crystallizing before the quartz. In the 
crushed granites quartz is frequently enclosed in feldspar, but 
this quartz is almost invariably of secondary origin. Another 
feature of the granite that needs mention is the large number 
of rock inclusions within it. The rock thus included is a fine 
porphyritic rock. Microscopic examination shows it to consist 
of a very fine ground mass almost wholly chloritized, within 
which occur porphyritic crystals of quartz and feldspar. These 
crystals vary in size and shape. Commonly they lack crystal 
faces and angles, and are round or sub-angular. The porphy¬ 
ritic crystals are largely quartz. Its structure and character 
indicate that it is a quartz porphyry. No trace of such a rock 
can be found in situ, fragments of it are abundant in the slate 
as well as in the granite. It doubtless belongs to a terrane 
which antedated the slate, from which the slate obtained a por¬ 
tion of its materials, and through which the granite was in¬ 
truded, bearing fragments of it upward. 
THE ARKOSE. 
Adjoining the granite and overlying it, on the west side of 
Conanicut proper, is a bed of arkose one hundred and ten feet 
thick. This bed consists of small rounded blue quartz grains 
imbedded in an argillaceous cement. The quartz of the arkose 
is identical with that of the granite, both in color and in the 
character of the inclusions. The cement is detrital feldspar 
containing a considerable percentage of muscovite. 
In passing from the granite to the arkose a gradual transi¬ 
tion is noticeable, the former at first firm and compact, becomes 
more and more disintegrated until finally it is loose incoherent 
arkose. The mode of occurrence is such as to leave no doubt 
that the arkose is a mantle of detritus arising from the secular 
decay of the granite. The outcrop of arkose at the shore is 
