INTEr.MEDIATE CHARACTER OF APPALACHIAN GRANITES 



15 



soda replaces jootash in the potassic feldspar, orthoclase, but anorthoclase, if 

 present, has not been identified. Perthitic gTowths and microcline are 

 usually present. 



In three of the analyses (table B), the percentage of NajO is greater than 

 that of K„0, but with only one exception (II), ISTaoO molecularly, either 

 exceeds or is approximately equal to K,0. 



Classification. — As is shown in table C, the total feldspar of the rocks 

 varies rather widely, from 42.07 to 72.76 per cent. In five of the analyses, 

 the calculated total feldspar is proportioned in the ratio of approximately 

 1 :1 of orthoclase to plagioelase molecules ; in VII and VIII plagioclase is 

 more than double orthoclase, and in I it is approximately four times greater. 

 In the old classification, on the basis of orthoclase-plagioclase ratio, II, III, 

 IV, V, and VII are quartz monzonites, VI and VIII granodiorites, and I 

 closely approaches the transitional position between granodiorite and quartz 

 diorite. Comparing the percentage of CaO in these, I, VI, and VIII, with 

 that of the other five analyses, it is found to be appreciably gTeater, and 

 falls well within the limits of this constituent for the granodiorites of the 

 Pacific Coast region, as defined by Lindgren.^ 



In the quantitative system of classification, the position of the Maryland 

 rocks has been computed as follows : 



Granites of Maryland. 



IVa. Granite from Dorsey Run cut, Howard county, Md. Not included in 

 tables of Maryland granites discussed in tliis paper. 



Most of the rocks represented in the above table (II, III, IV, V, and 

 VII) are sodipotassic, wliile the remaining ones (I, IVa, and VI) are 

 dosodic. 



"Lindgren, W. Op. cit., p. 275. 



