F. KTJTLEY ON FTTLGUKITE FROM MONT BLANC. 153 



to the rock quite close together, while from them a series of minute 

 globules of glass appear to be scattered in a somewhat radiating 

 manner, some of them being connected with the larger globules by 

 a thin fretted trail on the surface of the rock, indicative of the 

 sputtering of the fused material from the point struck by the 

 lightning (PL III. fig. 3). The blow-holes of the glass vesicles 

 from which the gas has escaped show an involution of the vesicle- 

 waU around them, as though each hole had been formed by a 

 wire pressed into the bubble while hot. This involution of the 

 vesicles around the blow-holes is possibly due to fusion, cooling and 

 contraction taking place with extraordinary rapidity. That the 

 cooling of the glass was unusually quick is proved by its freedom 

 from crystallites. Fulgurites, on this account, are probably the 

 purest natural glasses ever formed. 



The edge of a thin splinter of the rock taken from this specimen 

 fused with intumesence, before the blow-pipe, to a dark glass similar 

 to that of the fulgurite-globules. 



The dark and the white glass globules do not, as a rule, appear to 

 have mixed. They are often distinct, even when in contact, showing 

 a clear line of demarcation between the two different kinds. Some 

 instances may, however, be seen in which the dark shades off into 

 the white glass. The fusion of both the hornblende and the felspar 

 must have been so instantaneous, and the cooling so unusually rapid, 

 that the fused surface of each crystal solidified almost exactly in situ, 

 except where a sputtering of the molten matter was caused, either 

 by the force of the electric shock or by sudden disengagement of gas 

 from the fused material ; for, in presence of such intense heat, the 

 difference in the respective fusibilities of the hornblende and felspar 

 cannot be taken into account. 



On showing one of these fulgurites to Prof. Judd, he thought it 

 might yet be possible to procure a microscopic section of the rock 

 with some of the fulgurite adhering to it. It is to his kindness in 

 making several preparations that I have been enabled to give the 

 drawing of the edge of one of the sections showing the fulgurite, 

 containing one or two gas-bubbles (J) fig. 5, PI. III.) attached to the 

 rock. Unfortunately at this point the gneiss lying immediately 

 beneath the fulgurite has some very opaque bands, which by reflected 

 light appear white, not more so, however, than in many other parts 

 of the section remote from the edge or original surface of the rock ; 

 and from this I think it may still be inferred that little or no 

 alteration has been effected beneath the surface by the lightning. 

 Some hornblende also underlies the fulgurite and still shows 

 dichroism. The section at all events demonstrates that the vitrifi- 

 cation has been purely superficial. The foliation just beneath the 

 fulgurite is perfectly distinct. Prom a microscopic examination of 

 those slides which were cut from a specimen containing coarse 

 foliations of felspar it is seen that the rock is hornblendic gneiss, 

 the constituents being chiefly hornblende, triclinic felspar, and 

 quartz. The hornblende is of a deep reddish-brown colour by 

 ordinary transmitted light, and occurs both in large crystals and 



