Fairbanks. | 



Analcite Diabase. 



^93 





J. 



II. 



SiO, 



50.55 



47-4' 



AlA 



20.48 



18.65 



Fe 2 O a 

 Fe( ) 



2.66 

 4.02 



1 



j" I 0.2 I 





7-3° 



7- '/ 



MgO 



4.24 



5.06 



K,0 



2.27 



2.06 



Na,0 



8-37 



4.9O 



so 3 







H.,0 



■44 



5.05 



CI 



Trace 







100.33 



IOO.52 



III. IV. 



43-175 58.74 



15.236 20.85 



7.607 4. 1 5 



2.668 



10.633 0.36 



5.810 0.22 



4.070 4.23 



5.680 9.72 



0.940 



3.571 1.82 



99.390 100.09 



It is very difficult to arrive at any conclusion as to the composi- 

 tion of the different components of Cuyamas dike from the analy- 

 sis. The rock is remarkable for the high percentage of alkalies and 

 alumina and the low percentage of calcium. The proportion of 

 soda is too great for even a soda feldspar, although the high 

 extinction angles forbid that supposition. The presence of so 

 much potash with no potash feldspar is another thing which it is 

 difficult to explain. If a portion of it is contained in the analcite, 

 that would favor the view of the derivation of that mineral from 

 nepheline, which always contains potash. The percentage of cal- 

 cium is too low for the estimated proportion of augite in the rock, 

 unless it be of very peculiar composition, to say nothing of that 

 required for the feldspars with so high an extinction angle. The 

 analysis given by Rohrbach is of 3. I OC Is! containing labradorite 

 feldspar. The amount of potash and calcium agrees very closely 

 with that of the Cuyamas dike, but the soda is less. The feldspar 

 in the Arkansas rock, according to Williams, lies between albite 

 and labradorite, although the extinction angles correspond to lab- 

 radorite. This rock' contains twenty-five per cent of eleolite, but 

 the analysis shows a comparatively small increase of soda over that 

 of the Cuyamas dike. Williams considers the rock closely allied to 

 the theralites in mineralogical composition. 



