628 



CROSS, IDDINGS, P IRS SON, WASHINGTON 



No. 



Si0 2 



Al,Og 



Fe 2 3 



FeO 



MgO 



CaO 



Na 2 



K 2 



H 2 0+ 



H o 0- 



C() 2 



TiO a 



37-90 

 13-17 



8.83 



8-37 

 9-50 

 10.75 

 2-35 

 2. 12 

 1 .40 



5-30 



43-65 

 11.48 

 6.32 

 8.00 

 7.92 

 14.00 

 2.28 

 i-5i 

 1 .00 



tr 

 4.00 



42-35 

 12.29 



3-89 



7-05 



13.09 



12.49 



2.74 



1 .04 



1.50 



■32 



1.82 



F 2 5 

 S0 3 

 S 

 Cr 2 O s 



v 2 o 3 



NiO 



MnO 



BaO 



SrO 



LLO 



99.69 



100. 16 



•99 

 •05 

 tr 

 . 10 

 .04 



•03 

 .21 

 . 10 

 .09 

 tr 



100. 19 



No. 1. Hornblendite, Brandberget, Gran, Norway. V. Schmelck, anal.; Brogger, 



Erupt. Gest. Kryst. Geb.,Vo\. Ill (1899), p. 93. 

 No. 2. Olivine-gabbro-diabase, Brandberget, Gran, Norway. V. Schmelck, anal.; 



Brogger, Quart. Jour. Geo/. Soc, Vol. L (1894), p. 19. 

 No. 3. Nephelite-basanite, Ciruella, Colfax Co., N. M. W. F. Hillebrand, anal.; Bull. 



168, U. S. G. S. (1900), p. 171. 



mately half, chiefly as augite, while in the last very little such 

 modification of the norm has taken place. 



It is possible to devise a method of expressing in the nomen- 

 clature relative amounts of critical minerals for a comparatively 

 simple case such as the one just given. But the problem 

 is more complicated when several of these minerals, as 

 hornblende, augite, and mica, are present at the same 

 time, which frequently happens. The development of one 

 modifies the maximum that may be attained by another, so that 

 the expression of the relative degrees of development of each is 

 a function of the others, and an exact expression in the nomen- 

 clature becomes extremely difficult, and is perhaps imprac- 

 ticable. 



It will often be found useful to be able to indicate the 

 presence of certain minerals in rock groups, when the relative 

 amount which determines the Order, or lesser division, is not 

 known or needful for the purpose in view. Thus we may want 

 to speak of the persalanes or the dosalanes which carry quartz 

 or nephelite, without specifying the relative amounts of these, 

 i. e., without making use of the ordinal divisions. In these 



