COARSELY SPHERULITIC ROCKS. 189 



of Digoed. The silica-percentage has again, however, been probably 

 raised bv the removal of bases in solution. The specific gravity 

 was 2-59. 



Silica 88-09 



Alumina 6*03 



Oxide of Iron 0-58 



Lime 0-28 



Magnesia 1*65 



g^^^^^ I 2-53 (by difference) 



Loss on ignition 0*84 



100-00 



De Lapparent *, again, has described a very striking pyromeride 

 from Bouley Bay, Jersey, the spherules, which exhibit the character- 

 istic infillingsof chalcedony and quartz, being 25 centim. (10 inches) 

 in diameter ; but there seems to be little doubt, from an examination 

 of specimens of several more modest varieties occurring in the same 

 neighbourhood, that this rock also must be numbered among the 

 ancient rhyolites f. Perlitic structure is, moreover, known to occur 

 in the matrix of pyromerides, as at the Rauhfels of Wuenheim J 

 and Gargalong near Frdjus §; and M. Michel Levy ||, while dis- 

 criminating between the characters of acid rocks of different 

 geological ages, points out, with Delesse, that there are considerable 

 resemblances between many of the ancient " porphyries " and the 

 rhyolites of Tertiary days. Whatever the issue in particular cases, 

 the study of the slow processes of change to which igneous masses, 

 in common with all other rocks, are subject, cannot fail to throw a 

 vast amount of light on relationships hitherto obscured. 



I gratefully record my indebtedness to Prof. Judd for kindly 

 given help and much illustrative material ; also to Mr. F. T. S. 

 Houghton, F.G.S., for guidance at Conway and for several micro- 

 scopic slides ; and to Mr. J. F. Brooks, who worked with me in the 

 Wrekin area, and who has checked some of the chemical results. 

 Very many of the microscopic preparations referred to have been 

 made in the Geological Laboratory of the IS'ormal School of Science and 

 Royal School of Mines, in which the work of analysis was also 

 carried out. 



* " !?sote sur les roches eruptives de I'ile de Jersey." Bull, de la Soc. geol. 

 de France, 3me serie, tome xii. p, 287. 



t Cf. T. Davies, " Old Ehyolites from Bouley Bay." Min. Mag. vol. iii. 

 (1880), p. 118. 



I Rosenbusch, ' Mikrosk. Physiographie ' (1877), Band ii. p. 83. Delesse, Mem. 

 de la Soc. geol. de France, 2me serie, tome iv. p. 309. 



§ Fouque and Levy, ' Min. micrograpbique,' plate xv. Levy, "Roches anciennes 

 acides." Bull, de la Soc. geol. de France, 3me serie, tome iii. pp. 224, 235. 



II Loc. cif. p. 226. 



