HOLLOW SPHERULITES IN ALTEEED EOCKS. 445 



latter case, as at Bouley Bay in Jersey, it will be difficult to decide 

 between infilled primary and secondary cavities. While I still 

 assign a spberulitic character to the outer coats of the bodies in the 

 Welsh ' nodular felsites,' and fail to find evidence of their contrac- 

 tile origin,^ I am glad to find myself now in accord with Prof. 

 Bonney in admitting that the central masses of quartz may 

 frequently have resulted from the infilling of original vesicles. 



EXPLANATION OF PLATE XII. 



[The numerators of the fractions expressing the degree of enlargement of the 

 figures represent in each case the magnifying-power of the objective used. All 

 the sections are from the obsidian of the Rocche Rosse, Lipari.] 



Fig. 1. Vesicles, with local development of fibrous ingrowths, and formation, in 

 the upper example, of brown spherulitic matter in the surrounding 

 glass. X—. 



2. Portion of edge of two confluent vesicles, showing conical and other 



structures. X -|-. 



3. Vesicle with brown spherulitic zone, and ingrowths bordered by tridy- 



mite. XHK 



4. Contiguous vesicles, with * bridges' of spherulitic matter and tridymite. 



X^. 



5. Spherulitic 'bridge' becoming connected with the walls of the vesicle 



by felspathic outgrowths. X ^-#^. 



DlSCFSSION^. 



Prof. Bonnet expressed his sense of the great value of the paper, 

 which required careful reading and study before it could be properly 

 discussed. He was himself quite of opinion that spherulites might 

 form in more than one way, but he doubted, in cases where the 

 glass of the rock seemed quite fresh, whether the hollow in a 

 spherulite could be the result of decomposition. Hence he thought 

 that hollow spherulites so formed would be exceptional. Also 

 he believed that, as a rule, a solid spherulite with concentric shells 

 was not partly formed in the glass and partly infilled afterwards, 

 but had always been solid, cracks and crystallization being related. 

 He thought this must be the explanation of spherulites he had 

 examined in Arran, Jersey, and elsewhere, and he had seen solid 

 spherulites in artificially melted tachylyte. 



Prof. JuDD congratulated the Authors on the clear and logical 

 character of their paper, and was glad to find that a closer agree- 

 ment was now being developed among those who had discussed the 

 problem of the origin of lithophyses. 



Gen. McMahon called attention to some of Prof. Cole's specimens, 

 and suggested that lavas showing, as they did, a strongly striped 

 appearance owing to lines of partially-filled vesicles between lines 

 of dark glass might, when buried under the surface of the earth 

 and subjected to the action of heated water and pressure, be con- 

 verted in the course of time into a crystalline schist. 



^ T. G-. Bonney, ' Nodular Felsites in the Bala Group of North Wales,' Quart. 

 Journ. Geol. Soc. vol. xxxviii. (1882) p. 295. 



Q. J. G. S. No. 191. 2 I 



