ALBITK. 



49 



It will be remembered that in the preliminary experiments (p. 28 

 et scq.) the heating curve of these natural feldspars did not show an 

 absorption of heat which we were able to detect ; our first step was, 

 therefore, to find out what manner of process it was by which a charge 

 of crystalline albitc or orthoclase became amorphous without leaving 

 a thermal record behind. 



We prepared a charge of albite glass from a previous melt powdered 

 to " 100-mesh." In this glass powder a small crystal fragment (per- 

 haps 2 x 5 x 10 mm.) from the same original specimen and, therefore, 

 of the same chemical composition, was embedded beside the thermo- 



10. 



1 1. 



Fig. 10. Albite crystal embedded in charge of powdered albite glass. 

 Fig. 11. Same after heating. 



element as indicated in fig. 10. This charge was heated slowly to 

 exactly 1200 , slowly cooled again, and several thin sections prepared 

 from the crystal fragment and its immediate neighborhood. What 

 the microscope showed can best be seen from the accompanying 

 illustrations (Plate XX) groups of crystal fragments of microscopic 

 size, preserving their original orientation (extinction) perfectlv, but 

 with narrow lanes of glass where cleavage and other cracks had been, 

 forming a perfect network without a trace of disarrangement. Con- 

 siderable melting had taken place but no flow. Neither had the 

 charge as a w r hole made any movement to take the form of the con- 

 taining vessel after sintering together (fig. 11). 



