228 
Collie—Geology of Conanicut Island , R. I. 
microscopic mineralogical and structural resemblance and are 
probably of the same approximate age. 
Dike No. 1 .—The first dike met with intersects the granite. 
Its trend is N. 23 E. and dip N. 73 W. Its width is 15 ft. 
The core of the dike is dark colored and contains a large pro¬ 
portion of biotite, arranged in parallel planes, so that the rock 
closely resembles a schist, at the contact with the granite the 
dike is light colored and very much decomposed. • Biotite is 
here replaced by muscovite, inclusions of granite are found in 
this altered zone. 
Microscopically the rock is porphyritic in structure. The 
ground mass is a fine grained aggregate of orthoclase with a few 
fragments of striated feldspars scattered here and there through 
it, especially microcline. The outlines of the fragments are ir¬ 
regular. A crystal face or angle is rarely seen. In this holo- 
crystalline granular mass are crystals of biotite, and sometimes 
magnetite and apatite as minute accessory crystals. The 
biotite occurs in two generations. First, as large irregular 
plates 3 cm. long by 2 cm. wide; second, as lath-shaped idio- 
morphically developed crystals, varying in length from. 3 cm. 
upward to 1.5 cm.; the width rarely exceeds 5 cm. The 
second generation are arranged in parallel layers beautifully ex¬ 
hibiting flow structure, this feature is shown notably at the 
contact with the granite, where the biotite closely follows in 
and out all the sinuosities and irregularities of the granite. 
In many cases fragments of granite have been torn off and now 
lie imbedded in the mass of the dike; the biotite is found bent 
and wrapped about these inclusions. 
Dike No. 2 .— This dike occurs at a point just east of Hull’s 
Cove. Its strike is N. 60° W., its dip 30° N., 50° E. It came 
up along the cleavage of the schists, which is here well devel¬ 
oped. Its width varies in different portions of its course, the 
average is 6 feet. The rock is broken up into irregular blocks 
by jointing. Slickensided surfaces are frequent, this faulting 
though extensive is not on a large scale, the throw being 
usually but a few inches. 
The dike is slightly folded, the axis of the folds are N. E. 
and S. W., the direction of the force was parallel to the strike 
