28 Descriptive Arrangement of Volcanic Rocks. 



Art. IV. — Descriptive Arrangement of Volcanic Rocks ; hij 

 G. PouLETT ScROPE, Esq., M. G. S. 



[Communicated by the Author, (o the English Journal of Science, &c. No. 42, 

 for July, 1826.] 



In the course of a series of investigations of the geology of 

 volcanic districts, the writer of this paper has met with great 

 inconvenience from the want of a fixed nomenclature and 

 mineralogical classification of this family of rocks. 



MM. Cordier and Fleurian de Bclievue, in two well known 

 memoirs,* proposed a systematic arrangement of volcanic 

 rocks on mineralogical principles ; which, however, has not 

 as yet got into general use, owing perhaps to some obvious 

 imperfections in the mode of arrangement. 



M. D'Aubuisson followed these writers in classing the py- 

 rogenous rocks into two main families, trachyte and basalt ; 

 according to the prevalence of felspar or augite in their com- 

 position, and these terms have since been generally adopted 

 on the continent. 



But of late great confusion hns been introduced into the 

 subject by the "determination of M. de Beaudant,t and after 

 him of M. de Humboldt,^ to confine the terms Trachyte and 

 Basalt to rocks of a particular age and position in the geolo- 

 gical series. The attempt has originated in an unfortunate 

 mistake of these distinguished geologists, who have been led 

 by their observations to presume, that rocks of the mineral 

 character of trachyte never occur superposed to their own 

 conglomerates, or to tertiary strata. That this notion is 

 false in fact, may be proved by numerous examples from the 

 Mont Dor, Cantal, and Italy. But, had it been true, still it 

 is by no means allowable to employ the mineralogical title of 

 a rock to designate its place in a geological series. This is 

 the more strange in the latter author, because he talks of 

 granites of different ages, of syenites and porphyries of prim- 

 itive and transition formations, &,c. ; and because he ever 

 expresses himself in these positive words ; "There are tra- 

 chytes, phonolites, basalts, obsidians, and perhtes, o{ differ- 

 ent ages, just as there are different formations of granite, 



* Cordier. Essai sur les Roches Pyrogenes de tous les Ages. — Tournal dc 

 Physique. Fleurian de Bellvue. — Journal de Physique, torn. Ixxxiv. 

 ] Beudant. Hongrie, torn. iii. 

 t Humboldt. Essai Geoligique. 



