ALBITE. 51 



through the slide on exposed edges. At 1125 (Plate XIX X 600) a 

 four hours' heating gave unmistakable glass in tiny pockets and lanes. 



The above experiments with the Cloudland albite were completed 

 before we obtained the Amelia County material, but the latter proved 

 to be so much nearer to the type of pure soda feldspar that nearly all 

 the experiments were repeated with it, except that the crystal blocks 

 were embedded in powdered crystals. We did not develop any new 

 fact, however; the effects noted above reappeared in the same order, 

 except perhaps that melting went on a little faster in the Amelia 

 County specimen. As much melting was found after one-half hour 

 at 1 200 with the Amelia County sample as the Cloudland (Mitchell 

 County) albite 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 a microcline melt which had been heated as high as 

 1400 for another purpose. Another which had reached nearly 

 1500 showed no microcline, but one or two minute quartz inclusions 

 still remained undissolved. 



We made a rough attempt to get a more tangible idea of the viscos- 

 ity of these feldspars at their melting temperature in the following 

 way: A long, slender sliver (perhaps 30 X 2 X 1 mm.) of albite and 

 one of microcline were chipped from larger portions, 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 were completely amorphous (melted), but retained 

 their position 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 temperature a platinum rod was in- 

 serted 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 (Plates XXIV, XXV, and XXVI) . It will be 

 noticed that the melting began on the convex surface, where the 



