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



orthoclase are rather common; the small plates of secondary musco- 

 vite in these show a tendency to a latticed arrangement. Rosiwal 

 measurements on the sections of the River Road granite gave figures 

 corresponding to 7.2 per cent of biotite and 7.4 per cent of muscovite 

 by volume. The River Road granite resembles that of Guilford, 

 Maryland, as described by Keyes. 



Composition. — Chemical analyses were made of specimens both 

 from the River Road quarry (5) and the Rodman Street opening (6), 

 and the results are given in table 2, together with an analysis of the 

 Guilford, Maryland, granite (7), described by Keyes. 



The two Washington muscovite-biotite granites closely resemble 

 each other, that of the River Road quarry being slightly higher in 

 both alkalies, and with inverse proportions of magnesia and lime. 

 In their general chemical characters, and in their modal details, they 

 are very like the common granites of New England, Maryland, Vir- 

 ginia, South Carolina, and Georgia.^ The Guilford granite is slightly 

 higher in silica and alumina,^ and lower in magnesia; it is probable 

 that this has a higher content of muscovite than the Washington 

 granites. 



Norm. — The norms, shown in table 4, call for no special comment. 

 As shown by the symbols the three granites fall in toscanose, the 

 subrang to which belong more of the igneous rocks than to any other, 

 and in which most of the Piedmont granites fall. 



QUARTZOSE PHASES 



The granite of the River Road quarry contains some small stringers 

 and veinlets of a white, finely granular, highly quartzose aplite, 

 with sharp boundaries between the veins and the granite. The 

 microscope shows that this material is composed almost wholly of 

 small, irregular, interlocking grains of quartz, with very few of ortho- 

 clase, and rare small muscovite flakes. The quartz and feldspar 

 grains show slight undulatory extinction. The analysis (8) of the 

 quartz aplite of one of these veinlets shows that it is almost pure 

 quartz, with about one per cent of feldspar and mica. This quartz 

 aplite closely resembles the northfieldite described by Emerson,^ 

 except that the latter is rather more coarse grained, contains more 

 feldspar and mica, and is more decidedly pegmatitic. A highly 



^ Cf. Chemical analyses of igneous rocks, 1884-1913. U. S. Geol. Survey Prof. Paper 

 99: 167-175. 



8 The higher alumina is caused, in part, by the non-determination of titanium and 

 phosphorus oxides. 



9 B. K. Emerson. Amer. Journ. Sci. 40: 212. 1915. 



