and other Rocks from Montana. 295 



It is evident that both of these analyses are made of mixed 

 material and equally plain that one of the constituents is 

 sodalite. 



In order to calculate approximately the composition of the 

 two minerals let us assume an analcite composed as follows : 



55SiO, 23A1 2 3 , 13-5Na 2 0( + K 2 0), 8-5H 2 0= 100, 



and a sodalite containing 37-5 per cent Si0 2 . From the silica 

 contents it follows then that (III) contains 76 per cent soda- 

 lite and 24 per cent analcite. Calculating further the remain- 

 ing elements and disregarding for the present the small quan- 

 tities of Fe, Ca and Mg, one obtains 



(37-50 Si0 2 ), 31-50 A1 2 3 , 22-20 Na,0( + K,0), 6-30 CI, 2-20 H o 

 = 99-50 



which, excepting the high percentage of water, is a normal 

 sodalite. 



On the other hand, assuming a sodalite composed as above 

 and an analcite containing 55 per cent Si0 2 , it follows that 

 (IV) contains 68 per cent analcite and 32 per cent sodalite, 

 and that the other constituents of the analcite are 



(55Si0 2 ), 22-04 A1 2 3 , 13-40 Na 2 0( + K 2 0), 9-37 H~ 2 = 99-81, 



which corresponds very closely to a normal analcite. It does 

 not seem possible to avoid the conclusion that a part of the 

 water belongs to the sodalite. Under the microscope the 

 sodalite appears perfectly pure and isotropic and no other 

 zeolites such as natrolite or hydronephelite are present. If all 

 the water belonged to the analcite it would have to contain 

 about 15 per cent H 2 0. The amount of H 2 in analcite is, 

 however, very constant and varies only between 8 and 9 per 

 cent. As a matter of fact, the majority of sodalite analyses 

 do contain a small amount of water, and it has been suggested 

 that a certain quantity of CI may be replaced by (OH). 



A quantitative separation of the rock was made and the 

 following figures obtained for which only an approximate cor- 

 rectness is claimed : 



66 feldspar. 

 23 hornblende. 



8 sodalite. 



3 analcite. 



100 



The proportion between albite and orthoclase could not be 

 correctly ascertained on account of their very intimate inter- 

 growth, but from the thin sections and from the separation it 

 was estimated that the rock might contain 50 per cent ortho- 



