4 Professor Forbes on the Geology of Auvergne. 



the phenomena of nature, would perhaps be too rash. Our ig- 

 norance is such, that direct comparison of observed effects of 

 known causes by experiment, and the observed effects of un- 

 Jcnozvfi causes in nature, can alone lead us to well-founded con- 

 clusions. Yet geologists scarcely ever think of such an appeal 

 to experiment. Excepting those of Sir James Hall and Mr 

 Gregory Watt, we can hardly quote an example.* 



I have perhaps said enough to shew why I conceive that geo- 

 logy ought sometimes to be treated as a branch of natural phi- 

 losophy ; and in the hope of contributing something, however 

 imperfect, towards what I consider the neglected part of the 

 science, I have from time to time made collections in connection 

 with some of the leading problems, for the solution. of which 

 geology must be ultimately indebted to natural philosophy. One 

 of these is the action of heat upon rocks, and another is the me- 

 chanical power of elevation which igneous rocks have occasion- 

 ally exercised, and its consequences. Without pretending to 

 have devoted much special time to the inquiry, I have directed 

 my journeys so as to obtain the means of inspecting at least some 

 of the most important sites of ascertained or suspected geological 

 convulsion : nor can any merely local convulsions be considered 

 of much value; it is by the comparison of many points that we 

 are enabled to draw, with some degree of probability, the gene- 

 ral (though as yet almost empirical) conclusions whicli geology, 

 in its present state, admits of. With this view I have examined 

 the trap-rocks of our own island, the ophites of the Pyrenees, and 

 the serpentines of Anglesea and the Lizard, — the porphyries of 

 Northern Italy, the granite veins of Mount''s Bay and Glen Tilt, 

 — the ancient volcanos of Auvergne, the Eifcl, the Siebenge- 

 birge, and of Rome, — and the modern volcano of Vesuvius. 

 Without proposing anything like an abstract of conclusions de- 

 rived from these various sources, I may be permitted to offer 

 some considerations of a general nature on two points, to which, 



• I must not be supposed to have lost sight of the experiments of Dr 

 Turner and Mr Karcourt in this country, and of Mitscherlich and Becquerel 

 (whose very important results have bj no means attained the celebrity which 

 they seem to deserve) on the Continent. These v, e do not ewe to geologists, 

 but to chemists and physic'er.s. 



