12 MR. F. RUTLEY ON THE SEQUENCE OF [Feb. 1894, 



the first of these cases which I have cited the pcrlitic structure is 

 present only in the vitreous portions of the section, and it has been 

 developed subsequently to the formation of the spherulites. In the 

 fine example of obsidian from Pilas, Jalisco, Mexico, described and 

 figured in the Journal of this Society, 1 the development of the 

 perlitic structure has also followed that of the spherulites. Of this 

 there can be no doubt. 



Being anxious to adduce an instance in which the order of suc- 

 cession is reversed, i. e. where the spheruliric has succeeded the 

 jDerlitic structure, a section of perlitic obsidian from Buschbad, 

 near Meissen, was selected. Here in the clearest manner a sphe- 

 rulite or a group of spherulites (PI. I. fig. 4) may be seen to have 

 formed across a perlitic crack, just as in the devitrified obsidian of 

 Long Sleddale (PI. I. fig. 2) ; and that the spherulites in the 

 Meissen rock are devitrification-products there can be no question, 

 since they often follow the perlitic fissures, are irregularly distributed, 

 and do not form streams. A section of a devitrified obsidian from 

 Boulay Bay, Jersey (PI. I. fig. 5), containing coarse spherulites, 

 shows well-marked perlitic structure in the once vitreous portions, 

 which are now microcrystalline in structure. The boundaries of 

 these microcrystalline grains sometimes abut against the perlitic 

 fissures and at others traverse them. The perlitic was in this 

 case developed before the microcrystalline structure. 



Re-examination of sections of the Long Sleddale rock shows me 

 no reason to alter the conclusions at which I formerly arrived, nor 

 do I doubt the value of perlitic structure as a means of recognizing 

 the originally vitreous character of the rock, or of that portion of 

 the rock in which it may happen to occur. Perlitic structure is 

 known to be developed in amorphous bodies, natural and artificial ; 

 and in no single instance, so far as I am aware, has it been detected 

 in those which can be proved to have been in a crystalline or micro- 

 crystalline condition at the time of consolidation. That the Long 

 Sleddale felsite may vary considerably in structure within a very 

 limited area is extremely probable, and it may be that the specimens 

 collected and examined by Messrs. Harker and Marr differed in 

 some respects from mine. The original section upon which my 

 statements were based is at their disposal and will, I trust, help to 

 demonstrate that perlitic structure is not developed in the spheru- 

 litic parts of a vitreous or of a once vitreous rock, but that a spherulitic 

 structure may be superinduced in a rock in which perlitic structure 

 has already been developed. In this case the perlitic structure 

 appears to traverse the spherulites. In reality it is traversed by 

 them. On the other hand, when spherulites have been developed in 

 parts of a vitreous rock and perlitic structure has been set up after- 

 wards, the latter will not transgress the boundaries of the spheru- 

 litic areas but will be restricted to the glassy portions of the rock. 

 Finally, cases may occur in which both structures have been simul- 



1 Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 530. 



