204 



ALBERT D. BROKAW AND LEON P. SMITH 



the ferric condition. That the removal of ferrous iron may take 

 place in the early stages of alteration has been commonly recog- 

 nized. 1 



TABLE II 



Gain 



Loss 



Gain 



Loss 



Si0 2 . . . . 



A1A . . 



FeA-- 



FeO 



MgO... 

 CaO. . . 

 Na 2 0. . . 



K 2 



H 2 0-. . 

 H 2 0+ . . 

 Ti0 2 . . . 

 MnO. . . 

 C0 2 . . . . 

 S 



Total. . . 



Total Fe 



45-io 



1752 



3.12 



6.99 



4.67 



17-5° 



2-39 



i-37 



.04 



.46 



•31 



•47 



none 



29. 22 



385 



4-54 



2.81 



12.63 



i-45 

 .87 



■49 

 8.83 



20 

 16 



07 



.08 



12.51 



17-52 



11.66 



none 



•31 



■34 



. 21 



.64 



2.36 



8.06 



.08 



tr. 



.28 



none 



3265 



8-54 



6.99 



436 



17. 16 



2.18 



•73 



2.32 

 7.60 



•23 

 • 47 



oi 



39 



53-95 



7.62 



405 



9-33 



1. 71 



Essentially the same relations are shown by the ''straight line" 

 diagrams of Mead. 2 In Fig. 4 the analyses of the altered portion 

 are compared with that of the fresh rock. The full line represents 

 analysis 2, and the broken line analysis 3. In general, 3 is merely 

 an accentuation of 2, except for potash and total iron, both of which 

 show a change of sign in the direction of change. It is apparent 

 that in the later stage of alteration the removal of bases continues, 

 and that it is out of proportion to the further removal of silica as 

 compared with 2. The retention of potash is by no means unusual. 



In Table III the analyses are recast to molecular proportions to 

 emphasize some of the chemical and mineralogical features. It is 

 to be noted that even in analysis 2 the amount of silica is insufficient 

 to combine with all of the alumina to form kaolin. The alumina : 

 silica ratios are as follows : 1,1:4.36; 2,1:1.57; 3,1:1.19. For ka- 

 olin 1 : 2 is required. On the extreme assumption that all of the silica 

 in 3 is present in the form of kaolin, the analysis may be said to 



1 C. K. Leith and W. J. Mead, Metamorphic Geology, p. 22, Henry Holt & Co. 

 (1915)- 



- \V. J. Mead, Econ. Geol., VII (1912), 141-44. 



