462 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 19 

 BIOTITE GRANITE (TILDBN STREET TYPE) 



Megascopic characters. — This granite is of a general light gray 

 color, almost white in the mass, and is generally quite coarse grained. 

 Glistening, black plates of biotite are very conspicuous, the thin 

 streaks being up to 1 or 2 cm. long. These may be quite uniformly 

 distributed, but without definite orientation, as in specimens from 

 Newark Street; or arranged in roughly parallel position, producing 

 a markedly foliated texture, as in the Tilden Street and Pierce Mill 

 Road quarries. This foliation becomes more intense in two old 

 quarries on Broad Branch Road, about 1.3 kilometers north of Pierce 

 Mill. The spur on which are situated the Geophysical Laboratory 

 and the Holy Cross Academy seems to be composed wholly of this 

 (slightly foliated) type. The weathering of this granite has been 

 described by Merrill in the works cited above. This biotite granite 

 appears to be the most abundant type in and around the District. 



Microscopic characters. — ^In thin section this type is seen to be com- 

 posed chiefly of untwinned, alkali feldspar and quartz, with sub- 

 ordinate oligoclase, epidote, biotite, and muscovite. There is no 

 hornblende, no allanite was seen, and there are rare, small grains 

 of magnetite and prisms of apatite. The texture is cataclastic, but 

 not uniformly so. Much of the area of the section is rather coarsely 

 granoblastic, composed almost wholly of irregular grains of alkali 

 feldspar and quartz, which show some undulatory extinction. A 

 few smaller anhedra of finely twinned oligoclase occur here and there, 

 and there is considerable brownish biotite in irregular shreds, with 

 an occasional small plate of muscovite, some of which is intergrown 

 with the biotite and is primary. These areas contain no epidote. 



Between these areas are streaks and patches of finer grained, crushed 

 material. This is mostly untwinned alkali feldspar, with fewer 

 grains of quartz and oligoclase, and a little biotite. These areas 

 contain considerable secondary epidote in the form of small, color- 

 less, euhedral or subhedral crystals and prisms, v/hich occur in the 

 feldspar and are frequently agglomerated into clusters. There is 

 also a little secondary muscovite in very small shreds and plates. 

 No allanite was seen with any of the epidote; this is somewhat 

 remarkable in view of its common occurrence in the Maryland granites 

 described by Keyes, who regards the epidote as primary. A large 

 number of traverses across thin sections of the Tilden Street granite, 

 including areas of both kinds of material, gave the mode shown in 

 table 1, in volumes and weights. 



