PLAGIOCLASE AND PYEOXENE. 399 



clase with nearly parallel orientation, and sometimes extends outward in 

 micrographic intergrowth with quartz in the maimer already described, the 

 feldspar substance of the intergrowth having the same orientation as that 

 of the inclosed sanidine. 



Some crystals of feldspar with rectangular outline or cleavage, which 

 appear to be orthoclastic, exhibit a very faint striation between crossed 

 nicols, which is not sufficiently distinct to be positively determined as mul- 

 tiple twinning, but which suggests the microcline twinning which Miigge 1 

 observed in the sanidine of certain trachytes of the Azores. An irregular 

 optical behavior between crossed nicols is sometimes observed, by which the 

 extinction of light is unevenly distributed over the section of the feldspar. 



PLAGIOCLASE. 



The phenocrysts of striated feldspar have much the same general 

 characters as those of sanidine, but they are usually smaller and exhibit a 

 great number of thin stria?, or lamella?, twinned after the albite law, rarely 

 after that of pericline. They are sometimes in Carlsbad twins. The optical 

 behavior indicates that they are oligoclase or albite. In a few instances 

 the symmetrical extinction angles indicate labradorite. Such crystals yield 

 square sections. They carry the same kinds of inclusions as sanidine, but 

 often a greater amount of them. Small rectangular glass inclusions occur 

 in abundance in some plagioclases, while others are honeycombed with 

 inclusions of groundmass which equal the feldspar in amount. Inclusions 

 of augite are more common than in sanidine, and less frequently those of 

 magnetite, zircon, and apatite. Plagioclase is often inclosed by sanidine, 

 as already remarked, which sometimes only forms a thin shell around a 

 portion of the plagioclase crystal, and ■ may take part in the micrographic 

 structure of the groundmass. But the plagioclase in these rhyolites has not 

 been observed to enter into micrographic intergrowth with or to inclose 

 quartz. 



PYROXENE. 



The ferromagnesian phenocrysts in all of the rhyo'lite of this region 

 belong to the pyroxene group, with the exception of a small amount of 

 microscopic biotite in the rhyolite of Glade Creek. Fayalite is a product 



' Miigge, 0., Petrographische Untersuchungen an Gesteinen vou den Azoren: Neues Jahrbuch 

 fur Mineral., 1883, vol. 2, pp. 189-244 (p. 201 ). 



' 



