Vol. 59.] IN GLASSY IGNEOUS ROCKS. 433 



and between crossed nicols it breaks up into the ' patchy ' type of 

 devitrification. 1 



In one or two slides (Bonne-Nuit Bay and Anne Port, Jersey) a 

 spherulitic structure is more or less perfectly developed, and the 

 spherulites are surrounded by the minute quartz-felspar inter- 

 growth. In the slice cut from the Bonne-Nuit Bay specimen the 

 spherulitic portion is barely perceptible. Both slices exhibit patchy 

 devitrification. 



In a paper on the ' Microchemistry of Cementation ' by Prof. J. 0. 

 Arnold, in the Journal of the Iron & Steel Institute, 2 a number of 

 plates are given showing close analogies to sundry rock-structures. 

 Two of these figures are of supersaturated steel in which the surplus 

 cementite occurs either as ' heavy streaks ' having a tendency to 

 form regular meshes, or in well-laminated patches. The ground- 

 mass of the bar consists of ' normal pearlite.' 3 The irregular 

 streaks mentioned bear a close resemblance to the streaked or 

 gnarled structure of such rhyolites and obsidians as those figured 

 by Mr. Rutley in the Quarterly Journal of this Society in 1881. 4 



Secondary Devitrification. 



The rock-structures produced by secondary devitrification may be 

 arranged under two heads : — 



(a) Those produced solely by such crystallization, and 



(6) Those in which secondary have been imposed on primary crystallizations. 



(a) — An examination of a pitchstone from Carlitz, and study of 

 numerous English examples in which the changes are much greater, 

 suggest that secondary devitrification begins by hydration of the 

 glass in the neighbourhood of perlitic cracks. 5 



The latter, outlined by a belt of greenish altered glass, are 

 familiar in most devitrified perlites. In addition, in some Hungarian 

 examples a feeble granulation can be faintly discerned between 

 crossed nicols, favouring the neighbourhood of perlitic cracks in its 

 distribution. The presence of water in the perlitic cracks has, no 

 doubt, been an active agent in producing the devitrification. 



Evidence that the glass was not homogeneous when it solidified 

 may be found in the variable proportions of the green hydrated 

 material, the dusty grains of felspar, and the clearer grains of quartz. 

 Small variations in the amount of the dusty substance (kaolin) 

 do not appear to affect the perfection of the granular mosaic ; if, 



1 Where such separation is apparent, it would seem that the body of the 

 rock solidified as a fine-grained eutectic, the time possibly being insufficient for 

 coarser structures to form. 



2 Vol. liv (1898) p. 185, pis. xiii, xiv, xvii, & xix. The last two are those 

 above referred to. 



3 Op. cit. p. 190. Cementite is a definite carbide of iron, Fe 3 C. Pearlite is 

 an eutectic mixture of ferrite (that is, of particles of nearly or quite pure 

 metallic iron) and cementite, the two being usually interlaminated. Pearlite 

 forms in slowly-cooled steels. See Ency. Brit. 9th ed. vol. xxix, p. 572. 



4 Vol. xxxvii, pp. 406 & 407. 



5 [On this point I feel uncertain. — T. G-. B.] 



