chap. ni. of the Tr achy tic Series. 79 



tension in volcanic rocks, during their consolidation, we 

 must admit in the case of the thin alternate layers of 

 obsidian and pumice described by Humboldt, and of 

 the small, flattened, crenulated air-cells in the lami- 

 nated rocks of Ascension ; for on no other principle can 

 we conceive why the confined vapours should through 

 their expansion form air-cells or fibres in separate, 

 parallel planes, instead of irregularly throughout the 

 mass. In Mr. Stokes' collection, I have seen a 

 beautiful example of this structure, in a specimen of 

 obsidian from Mexico, which is shaded and zoned, like 

 the finest agate, with numerous, straight parallel layers, 

 more or less opaque and white, or almost perfectly 

 glassy ; the degree of opacity and glassiness depending 

 on the number of microscopically minute, flattened 

 air-cells ; in this case, it is scarcely possible to doubt 

 but that the mass, to which the fragment belonged, 

 must have been subjected to some, probably prolonged, 

 action, causing the tension slightly to vary in the suc- 

 cessive planes. 



Several causes appear capable of producing zones of 

 different tension, in masses semi-liquefied by heat. In 

 a fragment of devitrified glass, I have observed layers 

 of sphserulites which appeared, from the manner in 

 which they were abruptly bent, to have been produced 

 by the simple contraction of the mass in the vessel, in 

 which it cooled. In certain dikes on mount Etna, de- 

 scribed by M. Elie de Beaumont, 1 as bordered by alter- 

 nating bands of scoriaceous and compact rock, one is 

 led to suppose that the stretching movement of the 

 surrounding strata, which originally produced the 

 fissures, continued whilst the injected rock remained 

 fluid. Guided, however, by Professor Forbes' 2 clear 



1 ' Mem. pour servir,' &c, torn. iv. p. 131. 



2 'Edinburgh New Phil. Journal,' 18-12, p. 350. 



