216 ME. J. PARKINSON ON THE HOLLOW SPHERULITES [May 1901^ 



IV. The Effect of Solfataric Action in the Canon of the 



Yellowstone. 



In a road-cutting within a few yards of the Upper Falls of 

 the Yellowstone is a dark chocolate-coloured rhyolite, streaked 

 with a brighter red, and containing many porphyritic crystals of 

 felspar and quartz. The rock possesses a fluxional structure and 

 numerous fragments attributable to flow-brecciation. In a thin 

 section it appears brown and opaque, with a very obscure spheru- 

 litic structure, together with a rather blotched appearance as though 

 the constituents had gathered themselves into a series of nodes and 

 irregular patches. These are separated by light-coloured streaks 

 consisting of tridymite and opacite. In a few yards the character 

 of the rock changes considerably. Small spherulites weather out 

 from the dark grey surface, but fracture discloses a reddish-purple 

 rock studded with porphyritic felspars, and especially noteworthy 

 for the milky- white cavernous patches spread through it. In a thin 

 section we see that no dividing line can be drawn between altered 

 and unaltered parts, so insidiously has the intruding silica permeated, 

 and so gradually does the change take place. The staining of the 

 rock, probably by iron, and the obvious silicification which it ha& 

 undergone, are no doubt due to the permeation of hot water charged 

 with silica, and apparently during this process parts of the rock 

 have been entirely removed. 



A specimen of uniform milky-wbite rock, containing light grey 

 fragments in such quantity as to recall an ash, was collected from 

 the crags overlooking the Great Falls of the Yellowstone. Within 

 a couple of yards we find pale grey spherulites about an inch in 

 diameter, which for the most part, at least, are not hollow. Pro- 

 bably, then, the rock is a rhyolite, and the fragments are due ta 

 flow-brecciation. A thin section shows that the structure of the 

 rock has been preserved, save for a slight indistinctness in the outline 

 of the fragments. More transparent patches, such as distinguished 

 former specimens, are not found, and streaks of opaline silica 

 are not uncommon. Between crossed nicols there is no action on 

 polarized light. 



A few slides of spherulites from a friable obsidian near the Upper 

 Falls of the Yellowstone have been prepared, and are of some 

 interest. Firstly, they are almost identical in form and structure 

 with their far older representatives of Boulay Bay, except for the 

 greater number of porphyritic crystals that they contain ; and 

 secondly, they bear no evidence of silicification or alteration.^ In a 

 thin section the type of radial growth strongly recalls that of Boulay 

 Bay and of the small blue spherulites of Obsidian Cliff. The slice 



^ This statement admits of some modification. Occasionally one finds an 

 oblong or irregular patch, more translucent than its surroundings, and dis- 

 tinguished by the absence, more or less complete, of the felspathic fibres, and 

 by the presence of a colourless almost isotropic substance which may be 

 tridymite. 



