INTEEilEDIATE CHAEACTEE OF APPALACHIAN GBANITES 11 



Chemically, the difference is very pronounced. In the Sierra iSI^evada 

 intermediate type of rock (granodiorite)^ the range in SiO^ is from 59 to 69 

 per cent.; CaO, 3 to 6.50 per cent., though the latter is rarely reached; and 

 in total alkalies from 4.92 to 7.13 per cent, and may notably exceed the 

 CaO, but never fall short of it more than one per cent. Lindgren states that 

 in the relation of K2O to NaoO, the latter is apt to predominate, ranging 

 from 2.50 to 4.50 per cent. 



In the southern and central Appalachians type of intermediate rocks, the 

 range in SiOj is notably higher, 66.01 to 77.19 per cent., with an average of 

 71.18 per cent. ; CaO, 0.48 to 4.89 per cent.,- with an average of 1.85 per 

 cent. The range in total alkalies is from 4.39 to 10.24 per cent., with an 

 average of 7.69 per cent., always greatly in excess of the CaO, the average 

 ratio of total alkalies to lime being 4 to 1. In the relation of K2O to Na^O, 

 the two are approximately equal, the former ranging from 1.48 to 5.63 per 

 cent., with an average of 3.87 per cent.; the latter from 1.95 to 5.09 per 

 cent., with an average of 3.82 per cent. 



The plagioclase in the western type is, according to Lindgren,' a cal- 

 careous oligoclase or an andesine, and is at least double the amount of 

 potassic feldspar. In the eastern type the plagioclase is an acid oligoclase 

 with usually some albite, rarely a calcareous oligoclase, and is usually less 

 than double the potassic feldspar, the average ratio of potassic feldspar to 

 sodic-lime feldspar being 1 :1.88. This difference in composition of the 

 plagioclase accounts principally for the difference in lime content, in the 

 rocks of the two areas. 



It will be readily observed from these comparisons that on the basis of 

 both mineral and chemical composition the intermediate type of rock for 

 the two regions is strikingly different; the eastern type shows stronger 

 granite affinities and the western type stronger quartz diorite affinities. 



The following table makes clear the above relations. It shows the limits 

 of variation and average composition, as indicated in silica, lime, soda, and 

 potash, of granodiorites and quartz-diorites of the western United States, 

 ajid of the granites of the southern and middle Appalachians. 



■Lindgren, W. Amer. Joum. Science, 1900, Vol. IX, p. 275. 



-Excessively high lime due to large amount of epidote present (Maryland 

 granites) and not reached in any other case; next highest is from Maryland, and 

 is 3.74 per cent. 



^Lindgren, W. Op. cit, p. 277. 



