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E. A. STEPHENSON 



are perfectly transparent, have an index close to but greater than 

 i . 565, an extinction angle of 43 , and are probably monoclinic. In 

 experiment No. 26 no evidence of any change appeared. In experi- 

 ment No. 27 icositetrahedra of analcite appeared, some free and 

 some including fragments of the feldspar. The hornblende in all 

 of this group of experiments was unchanged. It is concluded that 

 the presence of the fluoride had practically no influence upon the 

 alteration. 



Group VI: Sodium bicarbonate solution upon feldspars and 

 hornblende. — These experiments embody an attempt to increase the 

 pressure both by raising the temperature and by increasing the 

 concentration of the carbon dioxide through dissociation of bicar- 

 bonates (Table VI). In experiment No. 32 scarcely any feldspar 



TABLE VI 



remained and much analcite appeared as free crystals and as aggre- 

 gates. Gelatinous silica appeared in the decanted solutions after 

 some time. Similar results with decided etching of the feldspar 

 grains appeared in No. 33 where the pressure was practically 

 doubled. Possibly less analcite formed in this than in the previous 

 experiment at the lower temperature. In No. 34 with albite, anal- 

 cite also formed with many needles like those in experiment No. 25, 

 Table V. In experiment No. 35 the tube burst where poorly sealed, 

 but the mineral was nearly all altered to needles as in experiments 

 Nos. 24, 25 and 34. These were analyzed qualitatively and found to 

 consist of soda, alumina, and silica and yielded no water in a closed 

 tube. The writer was not able to find any natural sodium alumi- 

 num silicate whose properties agree with these. Needles having 



