364 J. F. KEMP — GRANITES OF ATLANTIC COAST 



granitic gneisses and strongly contrasted with them. Thc}^ occur at 

 several points in the township of Westerly, Rhode Island, in its north- 

 ern portion. One of them is illustrated in i)late 38, figure 2. A good 

 exposure is shown in the village of Stonington, Connecticut, wliere the 

 eastern branch track to the steamboat landing leaves the main line. 

 Another one occurs in the railroad cuts between Stou}'- creek, Connecti- 

 cut, and Leets island. Additional occurrences are noted by Percival in 

 still other places. 



In thin-section it is seen that hornblende of a greenish brown color 

 makes up half the rock or more. A little biotite ma}'' accompany it. 

 The prevailing feldspar is plagioclase, but orthoclase and even a little 

 quartz are not entirely lacking. Titanite is a frequent and characteristic 

 accessory. Magnetite and apatite do not fail. The exposure near 

 Stony creek contains garnets that are full of inclusions. These rocks 

 have a pronounced foliation, and whether the}^ are considered basic 

 segregations in the original, more acidic magma that has yielded the 

 granitic rocks from which the prevailing gneisses have been derived, or 

 whether they are regarded as later basic intrusions or some still different 

 original, certain it is that they have passed through the same dynamic 

 metamorphism as the gneisses. Their chief interest lies in the fact that 

 they are met as inclusions in the massive granites and are one of the 

 most significant phenomena connected with them. 



The strike of the gneisses varies more or less, but it is prevailingly 

 northeast or northwest. 



The Granites 



distribution 



The several places described in the following pages are shown on the 

 map, plate 35. Intrusive granites have long been known and recorded 

 on Newport neck and Conanicut island, Rhode Island. The latter oc- 

 currence has been most fully described b}' Pirsson,* who regards the 

 granite as intrusive in Carboniferous slates, in which it has developed 

 some ver}^ interesting contact zones. The granite is a coarse porphy- 

 ritic variety and contains quartz, orthoclase, oligoclase, titanite, mag- 

 netite, zircon, apatite and secondary chlorite, epidote, sericite, and cal- 

 cite. It is so badly altered that the original ferro-magnesian mineral 

 could not be identified with certainty, but it Avas presumabl}' biotite, 

 with the possibility that some hornblende was also ])resent. A chem- 

 ical analysis yielded the following results :■ 



SiOo TiOo AI0O3 FeoOs FeO .AInO MitO CaO N;x..O KnO HoO Sp. gr. Total. 



71.23 0.21 13.64 1.70 1.00 0.05 0.75 2.31 3.55 3.79 1.72 2.69 99.95 



* L. v. Pirssoa : On the geology and petrography oC ronauicut island, Rhode Island, .\mer- 

 Jour. Sci., Novenjber, 1893, p. 363, 



