Professor Forbes on the Geology of Auvergne, ^ 



in a recent excursion to Auvergne, my attention was more par- 

 ticularly directed : 1. The igneous character of the trap rocks ; 

 2, The theory of elevation of Von Buch and Elie de Beaumont. 



I. There is no better proof of the necessity of a pretty exten- 

 sive induction in great geological questions than liic alternate 

 light and obscurity which the comparison of a new country with 

 one little known affords. Even the most skilful analysis of one 

 country or mountain group cannot make up for the extension of 

 ideas which the examination of several affords. For example, 

 were we to draw our conclusions as to the nature of granite 

 from the single study of the British isles, of the Alps, or of 

 the Pyrenees singly, we should certainly arrive at very con- 

 siderably different results. Still more so if we have to compare 

 the sedimentary deposits of those three very different localities, 

 as the lias of the Bernese Oberland with that of Bath, or the 

 mineralogical character of chalk on the coast of Kent and at the 

 summit of the Mont Perdu. It was not the want of talent, 

 but the want of extended observation, which led Werner into 

 his greatest errors, and it is to the same cause that many of his 

 followers have been led to support some of his least tenable 

 opinions, as those respecting the origin of the trap rocks, simply 

 by confining their attention to a single district. No one will 

 deny that the origin of our Scotch trap rocks might be yet in- 

 volved in great doubt, had we not indubitable specimens of 

 igneous action wherewith to compare them : could we not com- 

 pare the analyses of modern lava and basalt, — their mineralogical 

 structure, their dykes and veins, and their enclosed minerals. 

 On all these points a direct •omparison was obtained, and to 

 those who had seen and examined both classes of phenomena, 

 the conclusion was irresistible. Still it required a certain effort 

 of abstraction to realise, amidst our trap formations, the recol- 

 lections of the torrefied flanks of Vesuvius or Etna, to trace the 

 analogy, and to pronounce upon the general identity of origin. 

 In Auvergne this effort of abstraction was spared. Both phe- 

 nomena are side by side, nor perhaps has the most determined 

 Neptunist ever retired from an examination of these Phlegraean 

 fields, without feeling his former faith shaken ; whilst the majo- 

 rity have read at once the recantation of their heresy, amidst the 

 scarcely slumbering fires of Clermont and Le Puy. (To this 



