STUDY OF STRUCTURE OF FULGURITES 687 



glass on thin edges. A. Wichmann' made the most careful 

 petrographic study of this and other fulgurites and distinguished 

 this as "entirely homogeneous glass," without any alteration of 

 the andesite at the sharply defined contact. He considered the 

 bubbles as vapor-cavities [^da?npf-poreii) , and gave a clear repre- 

 sentation of the structure of a lightning-tube in a well-drawn 

 cross-section. The results of my own examination, which follow, 

 differ in some important particulars. 



The augite-andesite of this peak, though consisting of fresh 

 plagioclase-feldspar, with some partly decayed augite, horn- 

 blende, orthoclase, and magnetite, is deeply disintegrated by 

 weathering, so that, in some blocks, its grains crumble readily 

 under pressure from one's fingers. It is also traversed, even in 

 the most solid material, by numerous cavities and channels, a 

 few millimeters in width, of most irregular form, crossing and 

 connecting with each other at intervals of a few centimeters. 

 Most of these passages, in the more decayed specimens I have 

 examined, in the collection of Dr. Hovey, are one or two centi- 

 meters in width, and often coated by the olive-green fulgurite 

 glass to a depth of i to 1.5"'"'; some are even solidly filled up 

 by the glass, with a cross-section of 5 to S"""" in diameter or 

 over. Other cavities of exactly the same form and size were 

 noticed, adjoining but not connecting, which are now and appear 

 always to have been entirely free from fulgurite. Most, if not 

 all of these, therefore, seem to be preexisting cavities, later 

 occupied in many cases by fulgurite. A confirmation of this is 

 found, by optical examination of the thin sections, in the absence 

 of undulatory extinction in the grains of minerals adjacent to 

 the fulgurite, i. e., the lack of any indication of strain likely to 

 have resulted from actual perforation of the rock by lightning. 

 On the other hand, certain other specimens in the collection 

 from the same peak are penetrated by cylindrical winding tubes, ^ 



^Ide>n., Vol. XXXV (1883), pp. 849-859. 



"The approximately cylindrical form of the latter, and their curved windings, 

 have been well represented by Rutley, from the fulgurite furrows on glaucophane- 

 schist at Monte Viso, Cottian Alps [loc. cit., plate). 



