34: UNIVEKSITY OF VIKGINIA PUBLICATIONS 



Seven of the 30 analyses show orthoclase to range from 10 to 17.8 per 

 cent., the normal range in this constituent for granodiorite.^ In the remain- 

 ing 23 analyses the range in orthoelase is from 30 to 47.3. Only one of the 

 rocks contains more than 40 per cent, of orthoelase, and according to Lind- 

 gren- is, under the old system of classification, the only one of the 30 rep- 

 resented that would fall within the limits of a true granite. 



WheTX the orthoclase-plagioclase ratio, computed from the normative 

 feldspar for each of the rocks represented by the 30 analyses, is considered, 

 the results are further emphasized and are of considerable interest. Of the 

 30 analyses, 6 show the ratio of orthoelase to plagioclase to be less than 1 :1, 

 ranging from 1:0.45 to 1:0.97; three have a ratio not exceeding 1:0.5, the 

 remaining three being approximately 1 :1. Eleven show a ratio greater than 

 1 :1 and not exceeding 1 :1.5, six, less than 1 :2 and greater than 1 :1.5, and 

 the remaining eight range from 1 :2+ to 1:5+. 



Under the new quantitative classification of igneous rocks'*, the rocks rep- 

 resented by the 30 analyses above'* correspond to the names found under the 

 subrangs magdeburgose (2), alaskose (4), alsbachose (1), omeose (1), lipa- 

 rose (6), toscanose (8), lassenose (5), and yellowstonose (3). They fall 

 in the class persalane, in the orders columbare and brittanare, chiefly the 

 latter, with one in the order belgare. Although considered to contain orfho- 

 clase, as the dominant feldspar, only 2 are prepotassic and 1 dopotassic, 17 

 being sodipotassic and 10 dosodic. 



Comparing the results of the old and new systems a fairly general agree- 

 ment for the rocks will be observed. The 3 analyses having a ratio not 

 exceeding 1:0.5 are prepotassic (1) and dopotassic (2), containing 47.3, 



'Lindgren, W. Op. eit., p. 279. 



=Idem, p. 279. 



^Cross, Iddings, Pirsson, and Washington. Quantitative Classification of Igneous 

 Eoclcs, Chicago, 1903. 



'Washington, H. S. Chemical Analyses of Igneous Rocks. Published from 1884 

 to 1900 with a critical discussion of the character and use of analyses, U. S. Geol. 

 Survey, Prof. Paper No. 14. 



