28 Descriptive Arrangement of Volcame Rocks. 
Art. IV.—Descripiive Arrangement of Volcanic Rocks ; 3 by 
G. Pouterr Scrore, Esq., M. G.S. 
(Communicated by the ee? to the English Journal of Science, &c. No. 42, 
- or July, 1826.] 
Iv the course of a series of investigations of the geology 4 
volcanic districts, the writer of this paper has met with grea 
inconvenience from the. want of a fixed nomenclature ad 
mineralogical classification of this family of rocks. 
MM. Cordier and Fileurian de Bellevue, in two well known 
muse ep eee! a systematic arrangement of agen 
rocks on minera principles ;_ | however, has not 
t into. general use, owing perhaps. to som sr sin 
perfections in t ae ot iran arrangement. — corre 
. D’Aubuisson followed these writers in classing the py- 
rogen ous rocks into two main families, co er and basalt ; 
Sccording: to the. prevalence of felspar or augite in their com- 
on, and these terms have since been generally adopted 
on the. continent, 
~ But of late great confusion has pose introduced into. the 
er by the getenniuacien of M. de Beaudant,{ and after 
of M. de Humboldt,t 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 distingulehed seclogisie: bo who have been led 
by their observations to presume, - at r nicks of the mineral 
character of trachyte never occur super o'their own 
conglomerates, or to tertiary rats. That notion is 
false in fact, may be proved by numerous examples from the 
Mont Dor, Sanat and Italy. - But, had it been true, stil it 
pss no means allowable to employ the mineralogical title of 
a tock to designate its place in a geological series. This is 
the ma Seairan ge in the latter author, because he talks of 
granites of dif erent ages, of syenites and porphyries of prim- 
sition formati c.; and because he ever 
expresses himself in these positive words; “ There are tra- 
Seton, phonelet basalts, obsidians, and perlites, of di iffer- 
ent ages, just as there are different formations of granite, 
: ae Essai s nes de tous les Ages.—Journal Jounal 
agile. = isurian ran do pi — Journal de Physique, eer Ixxxiv. 
m. iii. . 
*} ee Bssai:-Geo eoligique, 
