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



amounts of diorite, gabbro, diabase, and pyroxenite) that extends 

 along the Piedmont Plateau from Georgia through Maryland, and 

 northward into New England. The granites and other igneous rocks 

 of the District have been little studied; or, at least, little detailed 

 information regarding them has been published. Merrill,- Williams,^ 

 and Keith^ give us but very summary descriptions, although Keyes^ 

 describes in great detail the related and closely similar granites of 

 central Maryland. Only one analysis, and that one incomplete, 

 of a fresh Washington granite has been published, together with 

 one of a weathered granite. 



Mr. L. H. Adams of this Laboratory selected a specimen of the 

 Tilden Street granite for his study of the compressibility of rocks. 

 I made a chemical analysis of this for him, and was thus led to analyze 

 other specimens from the District. The results of these analyses, 

 accompanied by brief petrographic descriptions, are given in this 

 paper, in the hope that they may be of use to some geologist who in 

 the future will study the region thoroughly. No attempt is made 

 here to discuss, even summarily, the structural geology or the 

 relations of the granites to the other igneous rocks or to the intruded 

 gneisses (fig. 1). 



Types. — Keith refers the granite of the Washington Folio to three 

 classes: granite-gneiss, granitic dikes in the gneiss, and intrusive 

 granite. All the specimens described in the present paper are of 

 intrusive granite, so far as has been ascertained, although some of 

 them are markedly foliated or gneissoid. Whether some of these 

 last belong in reality to the granite-gneiss or to the Carolina gneiss 

 must be left to the structural investigator. 



The granites studied may be referred to two fairly distinct types, 

 biotite granite and muscovite-biotite granite. Although all the 

 specimens of granite show evidence of crushing and other results 

 of pressure, non-foliated or slightly foliated forms of each occur, 

 and from these they pass into intensely foliated, gneissoid forms. 

 It would appear that the two extremes of the types are quite distinct 

 chemically, the type with muscovite being considerably higher in 

 silica than the type with biotite alone, but that they are connected 

 by intermediate forms. 



2 G. P. Merrill. Bull. Geol. Soc. Amer. 6: 321. 1895. Rocks, rock-weathering and 

 soils, p. 206. 1897. 



3 G. H. WiLLL\MS. U. S. Geol. Survey Ann. Rep. 15: 657-684. 1895. 

 *A. Keith. U. S. Geol. Survey Folio 70 (Washington Folio). 1901. 

 5 C. R. KeyES. U. S. Geol. Survey Ann. Rep 15: 685-737. 1895. 



