nCRLITIC OIJSIUIAX FKOM TILAS, MKXICO. 531 



X In addition to the ordinary cin-umfcrcntiiil })erlitic fissures, radial 

 fissures may souietinies be observed, but they do not pass uninter- 

 ruptedly through a series of circumferential cracks, being merely 

 continuous from one crack to the next. 



The dark brownish-red spherulites a])pear reddish or yellowish 

 brown in thin section, when viewed in transmitted light. Their 

 outlines ar(> sharply defined, and the perlitic structure accommodates 

 itself to their boundaries. From, this, and from the fact that no 

 trace of perlitic structure is to be discovered within them, it is 

 evident that the spherulites were formed before the perlitic structure 

 was develo])cd. It might be urged that molecular rearrangement 

 (induced during the formation of the spherulites) might have 

 obliterated any perlitic structure, had such existed, in the areas now 

 occupied by those bodies ; but such an hypothesis cannot be enter- 

 tained, since the delicate fluxion-banding of the obsidian is clearly 

 visible — passing uninterruptedly through the spherulites ; and it is 

 evident that, since this structure is preserved, so also would the 

 perlitic structure have been, had it been developed prior to the for- 

 mation of the spherulites. 



The sequence of the structures in this rock admits of no question. 

 The fluxion-banding w^as developed first, the spherulites were sub- 

 sequently formed, then perlitic structure was set up, and these fissures 

 were finally scaled by the introduction of chalcedonic matter. 



At the point or points (for there are sometimes more than one) 

 from which the spherulites originated, a confused microcrystalline 

 structure may usually be seen, and from this point, or from these 

 points, divergent bundles of delicate fibres or crystalline rods have 

 been developed. Within these fasciculi, finely puckered or vravy 

 transverse banding may be noticed, indicating slight pauses in the 

 crystalline growth, and occasionally the fluxion-bands appear to have 

 offered a temporary check to the development of crystalline bundles 

 which were growing approximately at right angles to the direction 

 of the fluxion-bands. In such cases the band which caused the 

 check has served as the base from which a fresh crop of crystalline 

 bundles has grown (as shown in PI. XYIII. fig. 3), and in one 

 instance the development of a spheruHte has been completely arrested 

 along a fluxion-band, as represented in PI. XVIII. fig. 4. 



In another case a spherulite has been developed prior to the for- 

 mation of a similar but larger spherulite which encloses it. Some 

 of the spherulites envelop s mall crystals of triclinic felspar, as shown 

 in the upper half of fig. 4, PI. XVIII. They present the appearance 

 either of imperfectly-developed or of corroded crystals, probably the 

 former, and delicate fringe-like processes from the enclosing sphe- 

 rulite may be seen penetrating them for a short distance beyond 

 their margins. 



Fig. 5, PI. XVIII., represents globulites, longulites, and minute 

 pellets of chalcedony occurring in the glassy portions of the rock. 



It seems very probable that this obsidian has been subjected to 

 hydro thermal agency since its solidification and the development of 

 its perlitic structure. The siliceous matter with which the perlitic 



Q. J. a. 8. No. 188. 1^ i' 



