GRANITES OF WASHINGTON 4G3 



100 100.0 



Composition. — Two specimens of this type were analysed. That 

 from the Tilden Street quarry (analysis (1) in table 2) was used by 

 Mr. L. H. Adams for the determination of its compressibiHty and 

 density. The density was found to be 2.739 at 30° C. The other 

 (2), was taken from a small quarry recently opened between Newark 

 and Ordway Streets near and west of Connecticut Avenue. This 

 is somewhat finer grained and less foliated than the Tilden Street 

 granite. An analysis (3), of the Broad Branch biotite granite, 

 described by Merrill, and one of the Rowlandsville, Maryland, granite 

 (4), described by Williams and by Keyes, are given for comparison. 

 (See first four analyses of table 2, and map, figure 1.) 



Norm. — These are analyses of rather "basic" granites, low in 

 silica, high in lime, and not high in alkalies, with a tendency to dom- 

 inant soda. Their chemico-mineral characters are well shown in 

 their norms, given in table 3, that of No. (3) being omitted, as the 

 analysis is incomplete. 



The resemblance between the three is manifest in the symbols. 

 All are near the border between classes I and II, all are in the quardo- 

 felic order and in the alkalicalcic rang. The two from Washington 

 are sodipotassic, but near the border of the dosodic subrang, while 

 that of Rowlandsville is dosodic. 



Comparing the norm of No. (1) with its mode, certain discrep- 

 ancies are cAddent. Much of the abundant anorthite of the norm en- 

 ters into the modal epidote, along with some of the ferric oxide. As 

 epidote has an excess of CaO over (Al,Fe)203 this will liberate some 

 alumina for the biotite, in which the normative hypersthene finds 

 place. The alkali feldspar is evidently a highly sodic orthoclase. 

 From the two sets of figures, however, even after all possible read- 

 justments have been made, it would appear that the sections used 

 for the Rosiwal determination of the mode were made in portions of 

 the rock which contained more alkali feldspar and less biotite than 



