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



That there has been a later intrusion of granite which has 

 necessarily metamorphosed the sedimentary strata and which 

 has been accompanied by the injection of dikes petrologically 

 related to the granite, and lastly the whole mass has suffered 

 from dynamic forces acting in a general north and south direc- 

 tion, the result of orographic movement and which has also 

 produced by shearing a mass of local grit. 



Note. — The author desires at this point to express his thanks 

 to Dr. A. E. Foerste who some years since made a brief study 

 of the island and has kindly given the writer a short abstract 

 of his notes made at that time. His outlining of the areas 

 and of the dikes is essentially the same as that here given. 

 From the presence of feldspar in the hornstones at one point 

 he is inclined to regard them as composed partly of altered 

 older basic eruptives, and the grit at Mackerel Cove he prefers 

 to correlate with the basal conglomerate of the basin, though 

 he regards it as derived from the granite. In other respects 

 the views of the writer are believed to be in agreement with 

 those expressed by Dr. Foerste. 



Petrography. 



Granite. — The Conanicut granite is a rather coarse grained 

 rock, on fresh surfaces of a light reddish color due to the iron 

 oxide developed in the feldspar. As remarked before it car- 

 ries nearly everywhere huge phenocrysts of orthoclase, exam- 

 ples 5 cm by 3 cm in width and thickness being a very common 

 size. It is therefore, strictly speaking, a granite porphyry, only 

 on a much larger scale of size of phenocryst and grain of 

 groundmass than usual. All of the main constituents may be 

 determined by the eye, magnetite, titanite, oligoclase, ortho- 

 clase and quartz, also dark masses of chlorite which are the 

 representatives of a former ferro-magnesian mineral. 



Under the microscope are found in addition zircon, apatite, 

 white mica and epidote, and it is seen that the rock even in 

 the freshest examples has suffered severely from dynamic pres- 

 sures ; the quartzes show undulatory extinction, the feldspars 

 are broken and filled with sericite. The large phenocrysts have 

 also suffered in this way and the cracks are filled with secondary 

 quartz and calcite. The ferro-magnesian mineral is no longer 

 recognizable, its place is entirely taken by chlorite, often mixed 

 with epidote. It is possible that hornblende as well as biotite 

 was originally present, the epidote as an alteration product and 

 the association with titanite rather favoring the idea. 



With regard to the order of crystallization of the various 

 components, the rock has in general been subjected to too 

 much alteration of various kinds that obscure the structure to 



