Vol. 59.] DEVITRIFICATION IN GLASSY IGNEOUS ROCKS. 429 



35. On Primary and Secondary Devitrification in Glassy Igneous 

 Rocks. By Prof. T. G. Bonney, D.Sc, LL.D., F.R.S., E.G.S., 

 and John Parkinson, Esq., B.A., E.G.S. (Read June 10th, 



1903.) 



[Plate XXVI.] 



A few prefatory words are needed in explanation of the form of 

 this paper. The authors have frequently discussed its subject, the 

 elder of them having kept it in view since 1877 l ; while the 

 younger has enjoyed favourable opportunities of studying large 

 spherulites, especially those in the obsidian of the Yellowstone, 

 which the other knows only from hand-specimens. When they had 

 agreed upon a joint paper, each wrote a draft, one of them having 

 undertaken to fuse them together. But he found this impractic- 

 able ; for, while their conclusions were practically identical, the 

 paths followed were very different, so the papers, after substituting 

 cross-references for some passages common to both, are now pre- 

 sented as separate chapters. 



Part I. — By John Parkinson, Esq., B.A., F.G.S. 



The excellent account and figures published in the Memoirs of 

 the United States Geological Survey by Prof. J. P. Iddings 2 render 

 superfluous any general description of the well-known spherulites 

 of Obsidian Cliff in the Yellowstone Park. Nevertheless some 

 mention, however brief, must be made of a few facts, as these are 

 closely connected with the general problem of devitrification. 



Excluding microliths, the first-formed crystallizations are the 

 ' granophyre-groups ' of Prof. Iddings. These are intergrowths of 

 felspar and quartz built with extreme delicacy, two or more crystals 

 of felspar entering into the composition and forming rectangular or 

 rudely spherical outlines. The greater the number of felspar-indi- 

 viduals the closer is the approximation obtained to a spherical form. 

 As Prof. Iddings states, it is clear that these microscopic ' grano- 

 phyre-groups,' together with the trichites and microliths, formed 

 before the lava came to rest. 



Crystallizations apparently of this type appear in most thin 

 sections of the obsidian, but their fibrous structure is barely capable 

 of resolution into components. The fibres are directed at right 

 angles to the containing surfaces. Crystallization then proceeded 

 on the more strictly spherulitic plan. According to Prof. Iddings 

 the first of this type to form were minute colourless spheres, ' their 

 finely-fibrous structure ' made evident only by the employment of 

 ' highly-converging light.' 



1 See his paper ' On certain Rock-Structures, as illustrated by Pitchstones 

 & Felsites in Arran ' Geol. Mag. 1877, p. 499. 



3 7th Ann. Eep. U.S. Geol. Surv. (1885-86) pp. 249-95 & pis. ix-xviii ; see 

 also Monogr. U.S. Geol. Surv. vol. xxxii (1899) pt. ii. 



Q.J.G.S. No. 236. 2i 



