Hyland — On some Spherulitic Rocks from Co. Down. 431 



groundmass consists of definite minerals, quartz and felspar, may- 

 be an original feature. Still, Professor Judd, in his recent paper 

 " On the Growth of Crystals in Igneous Rocks after Consolida- 

 tion," 1 promises to show in a future communication every grada- 

 tion from a glassy groundmass to one characteristic of the so-called 

 granophyric rocks, and to prove that the characteristic structures of 

 these rocks must he attributed to secondary rather than to primary 

 causes. 2 The double-refraction of the matrix, which felsites 

 and felsite-porphyries exhibit, may be either original or secondary ; 

 but we cannot as yet dogmatise upon the point. Where perlitic 

 structure is present, we may conclude' with Allport 3 and others 

 that the rocks which show this structure were originally in the 

 glassy condition. So far as we know, this structure can only arise 

 during the consolidation of a glass. In the absence of perlite or 

 some other feature equally characteristic of glass, we have no test 

 by which we can distinguish an original micro- or crypto- crystalline 

 groundmass from one which has been produced by subsequent 

 devitrification. 4 A special type of devitrification is the spherulite 

 which Rutley has described as occurring in a perlitic felsite from 

 the Long Sleddale Valley in the Lake District of England. 5 The 

 same author has, in conjunction with Hermann, studied the micro- 

 scopical characters of glass devitrified by artificial means, and 

 drawn attention to the conditions under which such devitrification 

 has taken place. 6 We may assume with considerable safety that 

 within certain limits there will be a more or less close analogy 

 between the results of the natural and artificial devitrification, 



1 Q. J. G. S., Lon., May, 1889, p. 175, et seq. 



2 I. c, p. 184. 



3 On Devitrified Pitchstones and Perlites of Shropshire, Q. J. G. S., Lon., xxxiii., 

 p. 449. 



4 Teall, on Some Quartz-felsites and Augite- granites from the Cheviot District, 

 Geol Mag. iii., vol. ii., 3, p. 110. 



3 Q. J. G. S., Lon., 1884, xl., p. 340. 



6 Herman and Eutley, On the Microscopic Characters of some Specimens of devit- 

 rified Glass ; with Notes on Certain Analogous Structures in Eocks : Proc. Eoy. Soc, 

 London, vol. xxxix., 1886, p. 87. 



See also Meunier Stan. : Experiences et observations sur les roches vitreuses. 

 Comptes rend, tome lxxxiii. 616-619 ; Eecherches sur la devitrification des roches 

 vitreuses. Compt. rend, lxxxiii., 1083-1085 ; Michel-Levy : Observations sur l'origine 

 des roches eruptives, vitreuses et cristallines. Compt rend, lxxxiii. 749-752. 



