PETROGRAPHY OF THE GRANITES 369 



biotite. It is regarded as xenotilne, which mineral Derby has shown by 

 concentrates to be widespread in many granites in Brazil. In some gen- 

 eral notes on the microscopic characters of biotite granites, G. P. Merrill* 

 remarked, in 1885, the richness of the Westerly granites in accessory min- 

 erals, and mentions fluorspar, sphene, menaccanite, magnetite, apatite, 

 epidote, and pyrite. The writer has also noted one or two irregular bits 

 of tourmaline in close association with biotite. Beryl is recorded from 

 Westerly ,t but it probably came from a pegmatite dike. Many of these 

 accessories are strongly suggestive of the presence of mineralizers in the 

 magma, and the remarks later made on the pegmatites at Westerly would 

 tend to corroborate the inference. 



Quonochontcmg orbicular granite. — The interesting orbicular granite at 

 Quonochontaug beach is a peculiar variation of the Westerly gray type. 

 It is associated with a massive variety that is much coarser than the gray 

 granite just described, but does not otherwise differ from it in mineralogy. 

 The details of the spheroids X have been set forth in the original citation 

 which is given above, page 365. 



The Stony Creek types. — The granites at Stony creek are of two varieties. 

 One, the Stony Creek red, is the characteristic building stone from this 

 locality. It is a coarsely crystalline rock that is in strong contrast with 

 the other types. The other, the Stony Creek gray, is obtained only from 

 the Brooklyn quarry. It is a more finely crystalline variety than the 

 last named, and has abundant garnets scattered through it, affording thus 

 the only instance of garnet met in the granites. Both these stones have 

 been extensively quarried, the former, however, more largely than the 

 latter. 



The Stony Creek red granite consists chiefly of large red microclines 

 which give it its characteristic shade. Tliey average more than 10 mil- 

 limeters in diameter under the microscope, are clear and but slightly 

 kaolinized. Quartz is abundant and oligoclase of a greenish white is in 

 variable quantit}^ It may sink to a small 2)ercentage, as is indicated by 

 the analysis by Professor L. P. Kinnicut later quoted. A little biotite is 

 present, but it is not specially notable in the stone. There is a little mag- 

 netite, and many zircons may be detected with the microscope. In pol- 



* G. P. Merrill : Report of building stones. Tenth Census, vol. x, p. 20. 



t A pamphlet entitled " Geology of Rhode Island," which was issued by the Franklin Society of 

 Providence in 1887, has but recently become accessible to the writer. It contains a list of min- 

 erals found at Westerly (p. 92) which embraces the following: "Feldspar crystals, micas, quartz, 

 amethyst, pyrite, ilmenite, beryl, garnet." While the pamphlet is largely a compilation, it gives 

 a very complete bibliography on the geology of the state and many notes of local interest. 



JThe writer has recently discovered that this peculiar granite is also mentioned in the report 

 to the Franklin Society cited in the last reference and that a cut of the granite is there reproduced 

 (see plate III and page 92.) The rock is regarded not unnaturally as " a kind of concretionary 

 conglomerate." 



LIU— Bull. Geol. Soc. Am., Vol. 10, 1898 



