682 A. A. JULIEN 



preliminary examination was made of a specimen of artificially 

 vitrified gneiss, with brown glassy fracture, obtained from the 

 hearth of a limekiln at Tuckahoe, N. Y. This had been evi- 

 dently thoroughly roasted and deprived of moisture but not 

 fused (as the gneiss structure still perfectly survived, in the 

 undisturbed parallel biotite-scales), saturated with slag from the 

 kiln, and finally thrown out and slowly cooled upon the refuse- 

 dump. In a thin section under the microscope, the results of 

 the action of fused glass upon the original minerals of the gneiss 

 were found to be as follows : a slight solvent attack upon the 

 biotite, rendering it in part opaque, with brown haloes and 

 cloudy wisps of iron oxide feathering out into the adjacent 

 glass, apparently by quiet molecular diffusion ; a decided but 

 partial solution of the quartz, of which a small portion survived 

 in angular remnants, still in place ; and complete solution and 

 disappearance of the feldspar. The main mass of the saturated 

 rock had thus been converted into pure colorless glass, with a 

 few coffee-colored stains, without any trace of crystallites or 

 new stony matter, notwithstanding its slow cooling. However, 

 globules of clear glass, about 0.5""" in diameter, were abundantly 

 interspersed, colorless like their matrix and with little adherence, 

 since many had become loosened and removed from the surface 

 of the thin section during the process of grinding. It was con- 

 sequently inferred : 



1. That minerals of acidic constitution, feldspar and quartz, 

 may be largely carried into solution, in contact with fused glass, 

 at a temperature below their thermal melting points. 



2. That their fused products tend to cohere in situ in sphe- 

 roidal globules, through possession of a different degree of 

 density, viscosity and contraction on cooling, from that of the 

 surrounding glass. 



3. That the process of devitrification has been prevented in 

 this vitrified gneiss by the peculiar constitution, probably the 

 acidity, of the unsaturated glass, and by an insufficient period of 

 cooling. 



In the development of a sand-fulgurite by almost instantaneous 



