NOV. 19, 1921 WASHINGTON : GRANITES OF WASHINGTON 465 



the larger and more representative amount of material used for the 

 analysis. 



MUSCOVITE-BIOTITE GRANITE (rIVER ROAD TYPE) 



A binary granite, containing about equal amounts of muscovite 

 and biotite, occurs in and near Washington, but is less abundant 

 than the biotite granite. Similar general quantitative relations 

 between the types seem to hold good in the Maryland granites as 

 described by Williams and Keyes, as well as in the granites of Georgia, 

 as described by Watson.*^ 



Megascopic characters .—The binary granite occurs in massive, 

 unfoliated form, as at the River Road quarry of the Bethesda Blue 

 Granite Company, about 1.3 km. northwest of the District line, in 

 Maryland, which is being actively worked. The rock of this quarry 

 may be regarded as typical. An intensely and finely foliated, gneissoid 

 form was collected by Dr. M. Aurousseau of this Laboratory at a small 

 opening on Connecticut Avenue, near the corner of Rodman Street. 

 This last was thought by him to be a highly foliated portion of the 

 granite exposed at Newark Street, about 0.3 kilometer to the south, 

 but the exact relations are unknown. 



The River Road granite is light gray and of finer grain than the 

 Tilden Street type. Small plates of both muscovite and biotite 

 are abundant ; they are arranged in small, irregular streaks and patches, 

 producing a slightly foliated structure. The Rodman Street specimen 

 is darker gray, much finer grained, and with a highly foliated, gneissoid 

 texture. 



Microscopic characters. — In thin section the two are much alike, 

 except for the distinctly foliated arrangement shown in sections of 

 the Rodman Street granite cut across the foliation. The texture 

 is highly cataclastic, both rocks giving evidence of intense crushing 

 and some recrv^stallization. Irregular, fragmental granules of quartz 

 and alkali feldspar are abundant, with few of finely twinned oligo- 

 clase. Irregular shreds of biotite, which is a decidedly greener brown 

 than in the other type, and tables of primary muscovite, are common. 

 The two micas are frequently intergrown, the muscovite being 

 generally included in the biotite, and these larger crystals of 

 muscovite are clearly primarv^ There is no amphibole nor magnetite, 

 and small grains of epidote are very rarely seen. Small irregular 

 areas composed of grains of quartz and highly crushed and sericitized 

 « T. L. Watson. Amer. Geol. 27: 199. 1901; Georgia Geol. Survey, Bull. 9-A. 1902. 



