Vol. 57.] OF THE YELLOWSTONE AND GKEAT BRITAIN. 223 



YIII. PONTESFORD HiLL (ShEOPSHIRE). 



The rock of the north-western corner of this hill is almost identical 

 with the rhyolite of Boulay Bay, and the nodules are quite indistin- 

 guishable from those of that rock.^ The quartz-amygdaloids are 

 rather irregular in shape, not infrequently stellate, and occasionally 

 show some relation to the periphery of the enclosing nodule. As at 

 Boulay Bay, lithophysae are not common, and occasionally radial 

 structure is not appreciable to a lens. The material in which the 

 nodules are embedded is frequently greenish-brown, presumably 

 once a glass, and contains small spherulites : it bears a very close 

 resemblance to the devitrified glass of Boulay Bay. In thin sections 

 we see that the nodules possess a well-marked spherulitic structure, 

 resembling that observed in the Boulay-Bay rocks, but, in the cases 

 examined, rather coarser than is usual in the Jersey examples. 

 Admirable examples of the tufted form of growth are common, and 

 these frequently project from part of the nodule into the quartz- 

 amygdaloid. The resemblance to the structures already described 

 from Boulay Bay and Wrockwardine is so close that no further 

 mention is necessary. 



IX. North Wales. 



I have examined the nodular rock at Beddgelert (in passing), at 

 Conway Mountain, and in the region of the Conway Falls and the 

 Lledr Valley. In a former communication ^ I have referred to 

 these rocks, and have nothing to add to what was then said^ In the 

 majority of cases, I believe that the conclusion brought forward by 

 Prof. Bonney holds good : — namely, contraction around a cavity.^ 



X. The Hypothesis of Corrosion. 



The continuity of lines of flow on either side of the star-like 

 elongations of a central cavity have been considered* as a clear 

 indication of corrosive action, and as a strong argument against the 

 former presence of a vesicle which would have diverted the lines of 

 flow. But we may also suppose that these star-like spaces arose 

 from contraction on cooling, and consequent cracking which caused 

 rupture without disturbing a structure previously acquired. In the 

 hollow spherulites of Obsidian Cliff, irregular vesicles, with an 

 inclination to be angular and even star-shaped, are far from 

 uncommon, and apparently quite analogous to those from the older 

 rocks. 



Another feature brought forward in favour of the hypothesis of 



^ I am indebted to the kindness of Mr. W. Boulton, AssocR.C.S., who is 

 engaged in a study of the rocks of this district, for permission to insert these 

 remarks on Pontesford Hill. 



'' Quart. Journ. Geol. Soc. vol. liv (1898) p. 112. 

 • 3 Ihid. vol. xxxviii (1882) p. 289. 



a. A. J. Cole, Quart. Journ. Geol. Soc. vol. xlii (1886) p. 183 & pi. ix, 

 fig 1 ; see also Miss 0. A. Raisin, ibid. vol. xlv (1889) p. 261. 



