STUDY OF STRUCTURE OF FULGURITES 69 1 



in embayments. Its origin is well shown next the Carlsbad- 

 twin of orthoclase (^-(?'), whose outer end ((^'), projecting into 

 the fulgurite current, has been shattered into this dust to a 

 depth of 0.1™™, seemingly after the adjoining fulgurite laminae 

 had somewhat consolidated. This and other feldspar-grains 

 lying along the edge of the stream, have been completely 

 encircled by fulgurite, with an inner film of angular dust, mark- 

 ing the outline of the minutely shattered surface of feldspar. 

 This micro-breccia is brought out between the crossed nicols in 

 distinct bright stripes. The phonolite groundmass generally 

 shows rather deeper embayments by fulgurite-attack than the 

 feldspar-grains ; but the hornblende-needles seem to lie entirely 

 unaffected, even in immediate contact {Ji) with the fulgurite- 

 stream. 



It is possible that the devitrified crust which constitutes this 

 fulgurite may be a remnant of an inner stony layer (as in Ful- 

 gurite III), from which a glass-coating may have been scaled 

 off during weathering ; but of the latter there is no evidence, 

 and the stony crust now occurs coated by small lichens. Yet 

 the peculiar structure of this crust, with its alternations of 

 micro-crystalline and of micro-brecciated laminae, signifies that 

 devitrification has not been a secondary process due to weather- 

 ing of a glass. 



RESUME. 



Considerable variation appears, in the characteristics of these 

 rock-fulgurites, from two probable causes : the variations in the 

 electric current, in regard to volume, intensity, duration, and a 

 probable series of successive discharges, in some cases, within 

 the same fulgurite (IV) ; and the difference of the rock-material 

 in the three specimens, gneiss, andesite, and phonolite. 



In regard to the duration of a lightning-flash, it is under- 

 stood not to have usually exceeded one hundred thousandth of 

 a second (Sylvanus Thompson). The discharge is known to be 

 oscillatory, surging back and forth, and even in many cases, like 

 that observed by Professor Hallock, apparently persistent and 

 continuous for some moments. Nevertheless, allowing for the 



