122 Day and Allen — Isomorphism and Thermal 



1200° with the Amelia County sample as the Cloudland 

 (Mitchell County) showed in the same time at 1225°, which is 

 readily enough explained by the relatively large quantity of 

 lime (anorthite) in the latter. 



Since both time and temperature enter into the delimitation 

 of the metastable region, further trials at temperatures above 

 1250° did not seem likely to add anything to the knowledge 

 already obtained. And if the heating were very rapid, the 

 temperature differences within the charge would be considerable. 

 A few isolated crystalline fragments were found in an ortho- 

 clase melt which had been heated as high as 1400° for another 

 purpose. Another which had reached nearly 1500° showed 

 no orthoclase, but one or two minute quartz inclusions still 

 remained undissolved. 



We made a rough attempt to get a more tangible idea of 

 the viscosity of these feldspars at their melting temperature in 

 the following way. A long, slender sliver (perhaps 30x2xl mm ) 

 of albite and one of microcline were chipped from larger por- 

 tions, spanned across small empty platinum crucibles, and 

 placed side by side in the furnace. These exposed crystals 

 were heated to 1225° for three hours. When removed, they 

 w T ere completely amorphous (melted) but retained their posi- 

 tion with hardly a trace of sagging. 



After this a number of similar slivers were prepared, 

 mounted in the same way, and heated to temperatures of from 

 1200° to 1300° for a few moments. At their highest tempera- 

 ture a platinum rod was inserted through a hole in the top of 

 the furnace and allowed to rest as a load upon the middle of 

 the crystal bridges. Under this load the partially melted 

 slivers gradually gave way, and were taken from the furnace 

 in the various forms shown in the illustrations. Slides cut 

 from these showed no squeezing out of the melted portion 

 between the crystal fragments on the side toward the center 

 of curvature, or open cracks on the outer side (Plate I, 7, 8.) 

 On the other hand, a variable extinction angle in an unbroken 

 crystal fragment frequently gave unmistakable evidence of 

 the bending of the crystal as well as the vitreous portion. 

 From these qualitative experiments it seems possible to assert 

 with confidence that the order of magnitude of the viscosity 

 of the molten portion (glass) is the same as the rigidity of 

 the crystals at these temperatures. 



Plate I, 7, shows a piece of Mitchell Co. albite heated to 

 1200° under load. The sagging is indicated by the curved 

 cleavage cracks. Plate I, 8, shows a sliver of microcline which 

 has been heated to 1200° under load. The sagging is shown 

 by the curvature of the crystal edges and the cleavage cracks. 



