370 L. V. Pirsson — Geology of Conanicut Island. 



dark color and much jointed showing macroscopically only by 

 color banding its sedimentary origin. Farther to the south- 

 ward the rocks become more greenish or grayish green in color 

 and more coarsely crystalline. In general it may be said that 

 as we approach the granite the hornstones are more crystallized, 

 harder, denser and break with a splintery fracture. 



As previously noted the sedimentary beds are thin, vary in 

 character and have a general !N. and S. strike, consequently as 

 one traverses the metamorphosed area in an E. and W. direction 

 along the line of the granite there is considerable variation in 

 the character and mineral constitution of the altered rocks as 

 will be shown later. Another striking feature in the meta- 

 morphic character of these rocks is the presence of great 

 quantities of fine veins of a yellowish color which traverse 

 them in all directions, and which on microscopic examination 

 are found to consist chiefly of tourmaline, of all minerals in 

 the contact zone perhaps one of the most characteristic of the 

 fumarole action of intrusive masses. 



Grit. — Besides the conglomerate already mentioned as occur- 

 ring at X and which is assumed as the lowest of the sedi- 

 mentary series on the island there occurs another conglomerate 

 rock or grit, exposed on the east shore of Mackerel Cove in 

 close connection with the granite and evidently derived from 

 it. It extends inland into C 4 as shown on the map : its out- 

 lines from a lack of good exposures are rather doubtful but 

 probably as given. It differs widely from the first mentioned 

 conglomerate in character in that it consists of fragmental 

 granitic material ; it is more of the nature of a grit consisting 

 of quartz grains and fragments mixed with decayed feldspar 

 material. Dale has characterized it as a " quartz clay aggre- 

 gate." Its derivation chiefly from the granite both from its 

 position and character of material seems quite clear. More- 

 over it contains fragments and even entire crystals of the large 

 orthoclase phenocrysts which form so striking a character of 

 the granite, their crystal form, broken or somewhat rounded. 

 Its derivation from the granite would make it more recent 

 than the adjoining sedimentaries and this is evinced by the 

 fact that it contains fragments of shales. Some of these near 

 the granite are slabs several feet in length whose long exten- 

 sion or strike is a little north of east and south of west, while 

 they are standing nearly perpendicular, the strike of the shales 

 to the north being N. and S. and the dip E. The granite 

 adjoining though much weathered has evidently been strongly 

 sheared and the whole appearance of this local mass of brec- 

 ciated material is strongly suggestive of its being the result of 

 pressure and shearing between the granite and the altered 

 conglomerates, shales, etc., at this point. 



