NOV. 19, 1921 WASHINGTON: GRANITBS OF WASHINGTON 4()7 



quartzose rock found by J. E. Spurr near Helvetia, Arizona, but as 

 yet undescribed, also is closely similar. ^° This was provisionally 

 named arizonite by Spurr and me, but Emerson's name, north- 

 fieldite, would seem to have priority. As pointed out by Emerson, 

 as well as by Spurr in the manuscript of his paper, such rocks are 

 to be considered as the ultra-silicic, hydrated portions of the magma, 

 which were the last to solidify in shrinkage cracks of the still hot 

 and recently solidified mass. 



GENERAI. REI.ATIONS 



Although this paper aims only to present the results of several 

 analyses of the granites of Washington, as a contribution to our scanty 

 knowledge of the petrology of the District of Columbia, yet a few 

 remarks may be permitted on the general relations of the igneous 

 rocks. The published data are very few, and my personal acquaint- 

 ance with the field geology is of the slightest. It would seem, how- 

 ever, that there is possibly an approach to regularity in the structural 

 arrangement of the igneous rocks of the batholithic (?) intrusive 

 mass. 



Referring to the Historical Geology Sheet of Folio 70, we see that 

 a broad band of "granite-gneiss" cuts obliquely across the western 

 portion of the area. This has a mode much like that of the River 



TABLE 4. Norms of Muscovite-Biotite Granite 



(5) (6) (7) 



Quartz 27.82 32.16 28.68 



Orthoclase 26.69 23.35 25.58 



Albite 31.96 * 28.82 33.01 



Anorthite 5.28 5.00 8.34 



Corundum 0.20 2.24 0.92 



Hypersthene 4.75 4.26 2.12 



Magnetite 2.32 1.86 0.93 



Ilmenite 0.61 1.52 



Apatite 0.67 0.34 



(5) River Road quarry. Symbol I".4.(l)2.3. 



(6) Rodman Street opening. Symbol I".(3)4."2.3. 



(7) Guilford, Maryland. Symbol 1.4.2.3". 



Road binary granite, but the chemical analysis ^^ of one specimen 



much resembles that of the biotite granite of Tilden Street. Toward 



the northern part of this gneiss (within the Washington quadrangle) 



are two long areas of biotite granite, with some muscovite-biotite 



'" An analysis of this is given in U. S. Geol. Survey Prof. Paper 99: 51. 



" Cf. G. H. Williams, op. cit. p. 670; also U. S. Geol. Survey Bull. 591: 49. 1915. 



