chap. in. Obsidian Formation. 7 1 



resemble those produced in glass, when slowl}- cooled. 

 In some fine specimens of partially devitrified glass, 

 in the possession of Mr. Stokes, the sphaarulites are 

 united into straight layers with even sides, parallel to 

 each other, and to one of the outer surfaces, exactly as 

 in the obsidian. These layers sometimes interbranch 

 and form loops ; bat I did not see any case of actual 

 intersection. They form the passage from the perfectly 

 glassy portions, to those nearly homogeneous and stony, 

 with only an obscure concretionary structure. In the 

 same specimen, also, sphaarulites differing slightly in 

 colour and in structure, occur embedded 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. Dar- 

 tigues, 1 in his curious paper on this subject, attributes 

 the production of sphasrulites in glass, to the different 

 ingredients obeying their own laws of attraction and 

 becoming aggregated. He is led to believe that this 

 takes place, from the difficulty in remelting sphserulitic 

 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. Fleurian de Bellevue 2 

 found that the sphserulitic 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 embedded. 



Comparison of the obsidian beds and alternating 

 strata of Ascension, with those of other countries. — 

 I have been struck with much surprise, how closely the 

 excellent description of the obsidian rocks of Hungary, 



1 « Journal de Physique,' ton. 59 (1804), pp. 10, 12. 



2 Idem, torn. 60 (1805), p. 118. 



