STUDY OF STRUCTURE OF FULGURITES 683 



fusion, it is unlikely that any selective power could have been 

 exerted among the sand-grains, through their slight variations 

 in fusibility, conductivity or other property. All within a radius 

 of a few millimeters were suddenly and completely fused, with 

 a limitation so sharply defined, in this fulgurite, that, in any 

 sand-grain, the quartz or feldspar may remain entirely unaltered 

 within an interval of about 0.02™" of the same material fused 

 and asfain devitrified. The molten mass of its wall became a 

 supersaturated solution of feldspar, with specially strong sol- 

 vency, however, toward any external quartz-grains within its 

 reach, through the tendency to form more acid silicate. Such 

 action probably accounts for the increased amount of silica 

 which has been determined in fulgurite-glass over that in the 

 original sand in several instances.' In this fulgurite also the 

 preponderance of feldspar over quartz in the attached sand- 

 grains may possibly have become exaggerated by the slower 

 solution of the former mineral. 



The view has been advanced ^ that a selective power of fusion 

 has been exerted by the electric current among the grains, but 

 without regard to their fusibility : the poorer conductors (quartz- 

 grains), offering so strong resistance as to become heated to the 

 point of fusion, those substances which are the best conductors 

 (iron-oxides and feldspars) escaping with least injury. It is 

 notable, however, that it is not a good conductor but quartz only 

 which has ever been recognized in remnants of grains inclosed 

 in the glass of other sand-fulgurites, and that no remnants what- 

 ever are enclosed within the glass of this fulgurite, apparently 

 because so rich in flux. 



Three rock-fulsfurites will now be described in the order of 

 increasing devitrification. 



' In a fulgurite from Union Grove, 111., silica 91.66 per cent, in the glass to 84.83 

 in the sand ; in another, 95.91 in the glass, to about 90 in the sand. (Merrill, 

 loc. cit., p. 85.) 



^Merrill, loc cit., p. 87. Diller notes, however, in a fulgurite on hypersthene- 

 basalt, that the sequence of alteration is according to degree of fusibility of the com- 

 ponents of the rock, greatest in the groundmass, then hypersthene, then feldspar, and 

 least in olivine. 



