3S4« Notices respecting Neia Books : — Darwin on the 



structure. In the same specimen, also, sphaerulites differing slightly in co- 

 lour and in structure occur imbedded close together. Considering these 

 facts, it is some confirmation of the view above given of the concretionary 

 origin of the obsidian and natural sphaerulites, to find that M. Dartigues*, 

 in his curious paper on this subject, attributes the production of sphaeru- 

 lites in glass to the different ingredients obeying their own laws of attrac- 

 tion and becoming aggregated. He is led to believe that this takes place, 

 from the difficulty in remelting sphaerulitic glass, without the whole be first 

 thoroughly pounded and mixed together ; and likewise from the fact, that 

 the change takes place most readily in glass composed of many ingredients. 

 In confirmation of M. Dartigues' view, I may remark, that M. Fleuriau de 

 Bellevuef found that the sphaerulitic portions of devitrified glass were 

 acted on both by nitric acid and under the blow-pipe, in a different manner 

 from the compact paste in which they were imbedded." 



The author next points out how closely the description of the ob- 

 sidian rocks of Hungary given by Beudant, that by Humboldt of 

 the same formation in Mexico and Peru, and likewise the descrip- 

 tions given by Scrope, Dolomleu, and D'Aubuisson, of the trachytic 

 regions in the Italian Islands, agree with his own observations at 

 Ascension. Many passages, he remarks, might have been transferred 

 without alteration from the works of those authors, and would have 

 been applicable to this island. He then proceeds to investigate the 

 lamination of volcanic rocks of the trachytic series, and after referring 

 to the frequently zoned structure of obsidian itself, and to the obser- 

 vations of the geologists named above, and the theoretical A'iews on 

 the subject enunciated by some of them, he finds, guided by Pro- 

 fessor Forbes's clear description of the zoned structure of glacier ice, 

 that 



" far the most probable explanation of the laminated structure of these 

 felspathic rocks appears to bCj, that they have been stretched whilst slowly 

 flowing onwards in a pasty condition J, in precisely the same manner as 

 Professor Forbes believes that the ice of moving glaciers is stretched and 

 fissured. In both cases, the zones may be compared to those in the finest 

 agates; in both, they extend in the direction in which the mass has flowed, 

 and those exposed on the surface are generally vertical : in the ice, the 

 porous laminae are rendered distinct by the subsequent congelation of in- 

 filtrated water, in the stony felspathic lavas, by subsequent crystalline and 

 concretionary action. The fragment of glassy obsidian in Mr. Stokes' col- 

 lection, which IS zoned with minute air-cells, must strikingly resemble, 

 judging from Professor Forbes' descriptions, a fragment of the zoned ice; 



♦ " Journ. de Physique, tom. lix. (1804), pp. 10, 12." 



t " Ibid. tom. Ix. (180.5), p. 418." 



X •' r presume that this is nearly the same explanation which Mr. Scrope 

 had in his mind, when he speaks (Geol. Trans, vol. ii. second series, p.228) 

 of the ribboned structure of his trachytic rocks having arisen from 'a linear 

 extension of the mass, while in a state of imperfect liquidity, coupled with 

 a concretionary process.'" — [On this subject we may refer for additional 

 illustration to the observations by Professors Forbes and L. Gordon in our 

 last Number, p. 206, on the motion of flowing pitch, as confirming the vis- 

 cous theory of glaciers. Those who are familiar with the aspect of masses 

 of glass, whether natural or artificial, which have retained their original 

 surface, will immediately recognise its identity with that of flowing pitch, 

 as represented in the lithograph illustrating the observations.] 



