682 



CROSS, ID DINGS, PIRSSON, WASHINGTON 



c) The process of calculating the norm from the mode, that is, 

 the standard mineral composition from a microscopical investi- 

 gation of rock sections, may be illustrated as follows : 



Two thin sections of Butte granite were examined ; one from 

 a surface excavation, twenty feet from the Parrott vein, very 

 fresh, with almost no signs of alteration, the other from similar 

 rock used in building in the city of Butte, the location of the 

 quarry not noted. 



In the first section an aggregate distance of 12,740 units of 

 the micrometer scale was measured, embracing 604 measure- 

 ments. The average diameter of the plagioclase crystals was 32 

 units, and the total length of plagioclase diameters was 5,492. 

 Table IX shows the lengths, relative volumes, assumed specific 

 gravities of each mineral, and the resulting percentages by weight. 



TABLE IX. 



Quartz .... 

 Orthoclase . 

 Plagioclase 



Biotite 



Hornblende 

 Pyroxene . . 

 Magnetite . 

 Pyrite 



First Rock. 



Total 

 Diameters. 



2,954 

 2,373 

 5,492 

 i, J 3o 

 482 

 252 



5i 

 6 



12,740 



Relative 

 Volumes. 



23-17 

 18.62 

 43.IO 



8.87 



3-78 



1.97 



.40 



.04 



99-95 



Sp. gr. 



2.65 



2-57 



2.68 

 3.00 

 3.20 

 3-30 

 5-17 

 5.00 



Weights. 



Second Rock. 



Relative 

 Volumes. 



20.97 



21 .07 



42.29 

 8.15 

 4.84 

 2.07 



•54 

 .02 



99-95 



Weights. 



20.34 



19.82 



4I.48 



8-94 



5-67 



2.50 



1.02 



.04 



99.81 



In the second rock section examined an aggregate distance 

 of 7,122 units of the scale was measured, embracing 403 measure- 

 ments. The average diameter of the plagioclase was 24 units, 

 and the total length of plagioclase diameters was 3,012. The 

 relative volumes and weights are given in the last two columns 

 of the Table. 



A comparison of the two cases shows how closely the rocks 

 resemble one another, the second being slightly higher in ortho- 

 clase and lower in quartz than the first. Other sections from 



