204 
Collie—Geology of Conanicut Island , B. I 
line of demarcation. The slate is so decayed that careful study 
does not reveal this structure so fully as is desirable. Secondary 
minerals, especially chlorite have almost wholly replaced the 
original constituents and the original character of the rock is 
not revealed in its entirety. The presence of these knoten-like 
bodies at the contact, the gradually increasing definiteness of 
these masses as the contact is approached, indicate beyond doubt 
that they are true contact phenomena. 
In certain localities the slate is conglomeratic in structure, 
it consists of brecciated material, a part of this material is slate 
closely resembling the country rock, the other part is a rock fel- 
Sitic in its appearance, clearly pyroclastic in origin. It is made up 
of lath-like crystals of feldspar, often oriented and indicating flow 
structure. In this ground mass occur porphyritic crystals of 
plagioclase and of ferro-magnesian minerals now so altered that 
they cannot be recognized. The presence of this porphyritic rock 
in the slate has lead some geologists to regard the slate as 
igneous in origin and it has been called a melaphyre. The field 
occurrence of the rock disproves such a conclusion, the breccia is 
clastic in its present form though some of its constituents are 
certainly volcanic in origin. The breccia is bound together by 
a coarse cement consisting of fragments of slate and volcanic 
rock, it resembles volcanic ash in part, consisting of broken 
fragments of rocks and broken fragments of crystals. The brec¬ 
ciated material is exposed so rarely and in such limited areas 
that nothing definite regarding its origin has been obtainable. 
The field occurrence of the slate resembles that of other slates in 
the Narragansett Basin known to be of Cambrian age. No 
fossils are found in the slate and therefore no attempt has been 
made to fix its geological horizon, other than to call attention 
to the similarity of its field relations with the slates farther 
north in this same area. 
THE GRANITE. 
Through the slate a coarsely textured, porphyritic granite has 
been erupted. It varies in macroscopic appearance and struct¬ 
ure. Normally it is dark red in color, and evenly textured. In 
the western portion of the area, where metamorphism has been 
