OF THE VITREOUS EOCKS OF MONTANA, U.S.A. 



397 



" the double refraction of compressed and suddenly cooled glass is 

 nevertheless essentially different from that of crystals ; " and he 

 adds that, in order to project the system of rings of a strained glass 

 disk upon a screen, using Dubosq's polarizing arrangement, it must 

 he placed at a point where " the rays by which it is struck are nearly 

 parallel, and traverse the plate in the same direction and with the 

 same length of path. The difference of path which gives rise to the 

 system of rings can therefore only be due to the fact that the double 

 refraction, whilst the course of the rays remains unaltered, increases 

 totvar ds the periphery of the plate. In a cr} r stal, on the contrary, 

 the double refraction is at all points the same for the same direction 

 of the rays." .... 



Before proceeding another step I wish to place side by side with 

 this statement an extract from the late Hermann Vogelsang's ' Kry- 

 stalliten ' *. Speaking of some crystallites in a piece of thick glass 

 from the glass-works at Stolberg, he states that for a certain distance 

 around these isotropic crystallites the glass exhibits double refrac- 

 tion, a neutral cross also traversing the anisotropic area. In one 

 case he noticed a slight disturbance of this cross in the neighbour- 

 hood of an ellipsoidal body, which seemed to him to indicate that 

 the strain in this instance was not constant in all directions ; and 

 he states, " It is also to be remarked that the glass at the boundary 

 of the polarization-picture is sometimes traversed by a fissure, a 

 spheroidally-running cleft. The polarizing action is not thereby 

 disturbed. Whether these cracks were produced during the cooling 

 of the glass, or whether they have subsequently been developed in 

 the splitting-off and grinding of the preparation, I am unable to say." 

 If the careful observer, who penned the lines I have just quoted, 

 had seen the preparation which is now placed before you, I think 

 that his doubts with regard to the origin of those spheroidal fissures 

 would have vanished. "We have here, I believe, additional confirma- 

 tion of the views advocated by Professor Bonney, Mr. Allport, Mr. 

 Cole, and myself with regard to the origin of perlitic structure. 

 But we have something more. If Vogelsang's crystallites be em- 

 bryonic crystals (as I think we may certainly assume that they are), 

 we have a close relationship between crystallogenesis and perlitic 

 fission ; and, indeed, in the section to which our attention is now 

 confined, there are plentiful examples of doubly refracting crystals 

 which are immediately surrounded by perlitic cracks, but which do 

 not, save very exceptionally, transgress those boundaries. 



In a paper upon a somewhat kindred subject f I have already 

 quoted a statement of Bischoff, to the effect that trachytic rocks, in 

 passing from a vitreous to a crystalline state, undergo a shrinkage 

 of nearly 10 per cent, of their original bulk. In those few instances 

 in which a perlitic crack passes through a crystal, there is commonly 

 another crystal developed by its side, which, with its surrounding 



* ' Die Krystalliten,' Bonn, 1875, p. 68. Admirable figures of these crystal- 

 lites are given in plates ix. and x. of the ibove work. 



f "On some Structures in Obsidian,^ erlite, and Leucite," Monthly Micro- 

 scopical Journal, vol. xv. p. 183. 



