204 HAROLD L. ALLING 
made by means of the extinction angles on the different faces, 
together with the indices of refraction. 
27. The application of these physical-chemical principles to the 
feldspar system furnishes, as a result, the means of solving practical 
geological problems. Two illustrations of the application of these 
conclusions to actual field problems are given on pages 275 to 
279 inclusive. 
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