NOV. 19, 1921 WASHINGTON: GRANITES OF WASHINGTON 4G9 



way, using the molecular numbers as ordinates and the molecular 

 silica percentages as abscissas; or still more clearly, by plotting the 

 relative percentages of the basic oxides (alumina to potash) taken 

 together against the silica percentages. The graphs for each constitu- 

 ent thus obtained show very regular decreases or increases, quite in 

 accordance with such observ^ations as have been made on rock series 

 from other regions. The points thus indicated furnish cur^^es that 

 need scarcely any "smoothing out," with the exceptions of certain 

 data for the Rodman Street granite and the Rock Creek Tunnel 

 hornblende diorite, some of the figures for which fall quite markedly 

 outside the smooth curves given by the others, indicating that they 

 are aberrant for some reason, such as possibly assimilation of intruded 

 gneiss. 



SUMMARY 



The igneous rocks of the District of Columbia are intruded into 

 Archean gneisses. Granites are most abundant, with small amounts 

 of diorite, gabbro, and pyroxenite. 



There are two types of granite: biotite granite (which seems to be 

 the more common"), and muscovite-biotite granite. Both contain 

 small amounts of oligoclase, but they differ chemically as well as 

 regards the kind of mica present, the biotite granite being lower in 

 silica and higher in lime. Transition forms probably occur. The 

 biotite granite has silica percentages from 67.5 to 69.0 while that of 

 the binary granite is about 72. Both types have undergone pressure, 

 giving rise to more or less well marked foliated textures, although no 

 very decided difference in chemical composition seems to be asso- 

 ciated with the foliation. Epidote is developed in the more intensely 



TABLE 5. Igneous Rocks of Washington, D. C, Arranged in - 

 Chemical Series 



Name Locality ^"p'relerence SiO= Symbol 



Binary granite River Road (5) 71.92 I".4.(l)2.3 



Binary granite Rodman vStreet (6) 71.66 I".(3)4."2.3 



Biotite granite Broad Branch (3) 69.33 (1)11.(3)4.3.3(4) 



Biotite granite Newark Street (2) 69.19 I(II).4. (2)3.3(4) 



Biotite granite Tilden Street (1) 67.46 (I)II.4."3.3(4) 



Biotite diorite Georgetown 56.41 II".4"."4.4 



Hornblende diorite Rock Creek Tunnel 56.18 "III.4.4.4 



crushed portions of the biotite granite, and this mineral is apparently 

 wholly secondary in origin. Epidote is very rare or absent in the 

 binary granite. No allanite, hornblende, or pyroxene was observed 

 in the Washington granites. 



