212 ME. J. PAEKINSOlf ON THE HOLLOW SPHERFLTTES [May I9OI, 



of the United States Geological Survey, vol. xxxii, pt.' ii, and to the 

 Geologic Atlas of the United States, folio 30. The latter contains 

 the Survey maps and a general description of the topographical and 

 geological features, together with a brief summary of the igneous 

 rocks by Prof. Iddings. 



I have studied these acid lavas at t^vo points : firstly, at the 

 well-known Obsidian Cliff, and, secondly, at the Canon of the 

 Yellowstone E-iver. The rocks at these two localities differ one 

 from the other in a most unusual manner. At Obsidian Cliff is 

 seen a magnificent section of an obsidian lava-flow, the lower 

 two-thirds columnar, the upper part crowded with large hollow 

 spherulites; at the Canon the eflect of solfataric action on the 

 rhyolite has converted the great thickness of these flows, which 

 now form the walls of the Canon, into the friable many-coloured 

 rocks which constitute one of the greatest attractions of the Park. 



[Part I — The Yellowstone.] 



II. Description^ of the Eocks of Obsidian Cliff. 



The glass. — At Obsidian Cliff this is black, but in other places, 

 as, for instance, locally at the Canon, a streaking together of red- 

 brown and black glass is not uncommon. Where spherulites abound, 

 the glass of course decreases in quantity, merely filling interstices, 

 and is occasionally almost entirely absent. In a thin section the 

 glass appears clear and transparent, and crowded with trichites 

 and microlites.^ It is too well known from the work of Prof. Iddinge 

 to require any description. 



The smaller spherulites admit of division into two groups: — 

 (i) This contains bluish-grey spherulites which are usually solid, 

 hard, and compact in texture, with a well-marked radial structure. 

 Not infrequently, the interior of the spherulite becomes hollow, and 

 the cavities of adjacent individuals communicate. These hollows 

 have a tendency to be stellate in outline, and not rarely the greatest 

 length is normal to the direction of the flow-band. Their walls are 

 usually formed of a narrow layer of brown earthy material, external 

 to which we find the harder bluish-grey spherulite. 



(ii) In this subdivision radial structure is almost entirely absent, 

 and cavities are invariably found. A thin border of whitish, 

 rather crumbly material around the cavity represents the spherulite, 

 and strings of such forms are embedded in the black glass. Occasion- 

 ally we find an approach to the harder blue spherulite of the fore- 

 going group, the more coherent material being external to the white 

 crumbly layer. Lines and bands of these spherulites are common 

 in the lower parts of the obsidian-columns, and often measure 

 only '025 inch across. The cavities are large in proportion to the 

 diameter of the spherulite, and are almost always arranged with 

 their longer axes normal to the direction of flow. When 'the 



1 See Iddings, 7th Ann. Eep. U.S. Geol. Surv. (1885-86) p. 273 & pi. xv. 



mi 



