Vol. 57.] SPHERULITES OF THE YELLOWSTONE AND GREAT BRITAIN. 211 



14. The Hollow Spherulites of the Yellowstone and Great 

 Britain. By John Parkinson, Esq., F.G.S. (Bead March 6th, 

 1901.) 



[Plate VIII.] 



Contents. „ 



rage 



I. Introduction 211 



[Part I— The Yellowstone.] 



II. Description of the Eocks of Obsidian Cliff 212 



III. Statement of the Problem 215 



IV. The Effect of Solfataric Action in the Caiion of the Yellow- 

 stone ; 216 



V. Conclusions in regard to the Yellowstone 217 



[Part II— Great Britain.] 



VI, Boulay Bay (Northern Jersey) 218 



VII. Wrockwardine (Shropshire) 221 



VIII. Pontesford Hill (Shropshire) 223 



IX, North Wales 223 



X. The Hypothesis of CoiTOsion 223 



XI. General Conclusions 224 



I. Introduction. 



In a paper which I had the honour of presenting to the Geological 

 Society some time ago on the Pyromerides of Bonlay Bay,^ I explained 

 the formation of these nodules by supposing the extrusive magma 

 in w^hich they were produced to be imperfectly mixed and in a 

 somewhat viscous state, a conclusion which facts in the field appear 

 amply to justify. These, and other nodules which I studied in 

 North Wales and near Wrockwardine, led me to suppose that the 

 dominant characteristic of these peculiar structures was not that 

 of spherulitic growth, which rather was secondary, and that the 

 principal cause operating in their formation, as just remarked, was 

 a clotting in the magma by a process of flow-brecciation. At the 

 same time, I did not suggest that this was the sole means by which 

 pyromerides were produced, and moreover I purposely left some of 

 the structures which are found, notably the not infrequent concentric 

 arcs of quartz, for later consideration and discussion. Eecently, in 

 crossing the American continent, I made a detour to the Yellowstone 

 Park with the object of studying the obsidian there, and it is the 

 result of this excursion which has led to the putting together of the 

 following notes. Since the paper above referred to was written, 

 I have examined the rocks of Boulay Bay and Wrockwardine for 

 the third and second times respectively, and have studied those of 

 Pontesford Hill. 



The general features of the National Park of the United States are 

 too well known to need more than brief mention ; I would, however, 

 refer to the recent monograph by Prof. Iddings in the Memoirs 



^ Quart. Journ. Geol. Soc, vol. liv (1898) p. 101. 



