IGNEOUS ROCKS 461 



the biotite granites cut slates of Oriskany (Lower Devonian) age, and 

 arkose derived from the same granites is found in the Horton series of 

 Pocono (Lower Mississippian) age at Horton Bluff. Therefore, the age 

 of these granites of the eastern Appalachians is inferred to be Middle 

 Devonian. 



LOWER PENN8YLVANIAN 



Diamond Hill felsite. — In the region about Diamond Hill, east of the 

 north-south fault which separates the biotite granite series from the Nar- 

 ragansett sedimentary series, is a large mass of felsite. Diamond Hill, 

 as will be shown below, forms a part of the felsite area and has been ex- 

 tensively replaced by silica. The extent of the felsite is 2 miles in length 

 and three-quarters of a mile in breadth. Another small occurrence of 

 similar felsite has been found just northeast of the area here mapped. 



The felsite is in general a fine-grained dense, dark green rock showing 

 occasional phenocrysts of quartz and feldspar. It is everywhere highly 

 altered, and in the vicinity of Diamond Hill quartz veins of microscopic 

 and megascopic size ramify it. In general, the felsite to the south is 

 lighter green in color than that to the north, and where it is partly re- 

 placed by silica, it is light brown in color, resemblng a fine-grained sand- 

 stone in appearance. The felsite just south of the Massachusetts-Ehode 

 Island State line resembles a green schist, it being very dark green in 

 color, with occasional patches of chlorite. It is cut by calcite veins which 

 vary in width up to one-eighth of an inch. 



The unsilicified felsite of Diamond Hill is a dark greenish rock of 

 aphanitic texture. Under the microscope it is seen to consist of a fine, 

 indefinite mixture of feldspar and quartz, chlorite, calcite, sericite, and 

 magnetite, sprinkled with grains of epidote. With this material are 

 grains and patches of coarser epidote and an occasional phenocryst of 

 plagioclase and quartz. The feldspar phenocrysts are badly altered and 

 can not be determined very accurately, but may be as basic as andesine. 

 A part of the quartz may be secondary. 



The felsite from near the Massachusetts-Rhode Island line consists of 

 feldspar and femic constituents or their alteration products and a minor 

 amount of quartz. The feldspar appears to be oligoclase. Epidote is not 

 present to any extent, but magnetite and calcite are, the chlorite often 

 occurring in patches evidently secondary after some original femic con- 

 stituent. Near the outcrop of this felsite is another of a light pinkish 

 color. It is essentially aphanitic in texture, but shows a few phenocrysts 

 of albite-oligoclase feldspar and quartz; also patches of chloritic material. 

 It is also considerably altered. 



