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



somewhat curved outcrops ; they wedge out in places along 

 the strike and are filled with veins and large lenticular masses 

 of secondary quartz. The conglomerate of the given section 

 contains quite large rounded pebbles in a fine grained cement ; 

 where its outcrop can be traced along the shore it is seen in 

 places to have suffered severely from dynamo-metamorphic 

 processes, the pebbles are flattened, the cement becomes mica- 

 ceous and transitions to gneiss or mica schist occur. It is 

 strikingly similar in lithologic characters to the drift bowlders 

 found over the lower part of the island and wholly different 

 from the conglomeratic grit next to the granite on the east 

 shore of Mackerel Cove in C 4. 



The granite of Conanicut is coarse grained of a mixed 

 grayish color on fresh exposures and it contains large. often 

 huge phenocrysts of orthoclase well distributed. These pheno- 

 crysts are of great importance in determining the' relations of 

 the granite and the grit in C 4. Petrographically the granite 

 is described later. As will be seen by reference to the map, 

 fig. 2, it forms the whole of the lower portion of the main 

 part of the island. Its subaqueous limits can be pretty well 

 defined by the sedimentaries on the west shore of Mackerel 

 Cove ; to the south by Kettlebottom Rock which is an out- 

 crop of shale rising some eight feet above tide. Attempts 

 w r ere made to examine this rock closely but the heavy seas 

 breaking on it prevented a landing; at a distance of some fifty 

 yards it could be easily seen to be of shales and it is not prob- 

 able that a drift mass could remain on a ledge in such an ex- 

 posed position. On the Newport shore to the S.W. the shales 

 recur again and the small island to the S.E. of Bull's Point in 

 Gr 8 is composed as to its north part of altered sedimentaries 

 and to the south of granite. 



Evidences of intrusion. — That the granite is intrusive is 

 best seen at Ball's Point where it is directly in contact with 

 the sedimentary beds here altered to a dense hornstone-like 

 rock. The granite is finer grained and more porphyritic ; the 

 contact between the two rocks is solid, specimens breaking 

 across both as if one rock : it has sent out apophyses in the 

 shape of dikes which cut into the sedimentary beds. Further 

 to the eastward in E 4 the contact is buried, but at one point 

 the granite and hornstone are but a few yards apart ; here the 

 granite is fine grained and aplitic in character, the hornstone 

 extremely dense and tough. A short distance further west- 

 ward between two little hills in E 4 there occurs an outcrop on 

 the east side and near the bottom of the westward hill of a 

 dark flinty rock, felsitic in appearance, characteristic of the 

 contact facies of rapidly chilled highly siliceous magmas and 

 which microscopically shows a crypto-crystalline groundmass 



