chap. in. Obsidian Formation, 69 



structure, concretions in sedimentary deposits, that one 

 is at once tempted to attribute to them an analogous 

 origin. They resemble ordinary concretions in the 

 following respects : in their external form, — in the union 

 of two or three, or of several, into an irregular mass, or 

 into an even-sided layer, — in the occasional intersection 

 of one such layer by another, as in the case of chalk- 

 flints, — in the presence of two or three kinds of nodules, 

 often close toget her, in the same basis, — in their fibrous, 

 radiating structure, with occasional hollows in their 

 centres, — in the co- existence of a laminary, concretionary, 

 and radiating structure, as is so well developed in the 

 concretions of magnesian limestone, described by Pro- 

 fessor Sedgwick. 1 Concretions in sedimentary deposits, 

 it is known, are due to the separation from the sur- 

 rounding mass of the whole or part of some mineral 

 substance, and its aggregation round certain points of 

 attraction. Guided by this fact, I have endeavoured 

 to discover whether obsidian and the spliEerulites (to 

 which may be added marekanite and pearlstone, both 

 of them occurring in nodular concretions in the trachytic 

 series) differ in their constituent parts, from the minerals 

 generally composing trachytic rocks. It appears from 

 three analyses, that obsidian contains on an average 76 

 per cent, of silica ; from one analysis, that spha3rulites 

 contain 79*12 ; from two, that marekanite contains 

 79*25 ; and from two other analyses, that pearlstone 

 contains 75*62 of silica. 2 Now, the constituent parts 

 of trachyte, as far as they can be distinguished, consist 

 of feldspar, containing 65*21 of silica ; or of albite, con- 

 taining 69*09 ; of hornblende, containing 55*27, 3 and of 



1 ' Geological Transactions,' vol. iii. part i. p. 37. 



2 The foregoing analyses are taken from Pendant, * Trait e de 

 Mineralogie,' torn. ii. p. 113 ; and one analysis of ob&idian, from 

 Phillips's ' Mineralogy.' 



3 These analyses are taken from Von Kobeli's ' Grundzuge der 

 Mineralogie,' 1838. 



