222 ME. J. PARKIXSOX ON THE HOLLOW SPHERTJLITES [May I9OI, 



The practical identity in the structure of the lithophysae from the 

 Yellowstone and Wrockwardine is still further emphasized by the 

 presence, in the latter, o£ the remnants of a mineral which closely 

 resembles tridymite in external form. It is rather sporadic in its 

 occurrence, but locally encrusts thickly the walls of the concentric 

 cavities of the spherulite. It is now replaced by quartz, so that 

 the original polarization cannot be determined.^ The outward form 

 of the original mineral can still be observed, owing to the presence 

 of some impurity in the infiltrating material which has preserved its 

 outline by a thin encrustation. Sections in one plane are lath- 

 shaped, measuring about '003 inch x '0006 inch, and in a plane 

 at right angles to the former the shape is that of a hexagon. The 

 complete hexagon has not been observed, two sides being absent 

 where the mineral joins the wall of the amygdaloid ; this is most 

 usually the case in the tridymite of the Yellowstone. Often only 

 two sides of the hexagon are seen. The general habit of the mineral 

 strongly recalls the encrusting tridymite of the Yellowstone litho- 

 physae, and there can be no doubt, at least, that it is not due to 

 deposition, in some unusual manner, by the infiltrating silica. 



One rather interesting point follows. In some of the rocks a thin 

 section reveals the presence of a number of incomplete spherulites, 

 often fragmentary in appearance, and surrounded by secondary 

 quartz. The structure is due partly to the presence of origiual 

 gas-vesicles, partly perhaps to movement subsequent to the 

 formation of the spheriilites, and partly to brecciation after 

 solidification. 



Sometimes, however, the outlines suggest some amount of corro- 

 sion. It is possible to fix a date for the latter, owing to the typical 

 development of the mineral resembling tridymite on such a corroded 

 surface. We may suppose that the circulation of heated waters 

 through the rock effected some corrosion, though not, I think, to 

 any great extent, and that subsequently, when the temperature fell 

 somewhat, tridymite, or a mineral closely resembliug it, was 

 deposited. (See fig. 2, III, p. 214.) 



The obsidian of the Yellowstone region contains so large a 

 number of porous spherulites, that in any investigation of older 

 lavas at all comparable with the American rock one must be 

 prepared to find indications, and probably abundant indications, of 

 these peculiar structures. 



The felspathic fibres, described and figured in the foregoing pages 

 as projecting into the cavities of the spherulites from the old 

 fiows at Boulay Eay and Shropshire, as well as the open, non- 

 compact nature of those parts of the spherulite which are adjacent 

 to such cavities, show how closely these older lavas approximated 

 to the younger in the conditions under which they solidified. The 

 formation of a mineral suspiciously like tridymite clearly points in 

 the same direction. 



^ The replacement of tridymite by quartz has been described by M. E. Mallard, 

 JBull. Soc. Min. France, toI. xiii (1890) p. 161, from the Euganean Hills. 



