538 HISTORY OF GEOLOGY. 



wick (not, however, is free from exception), that in one particulai 

 class of rocks the slaty Cleavage never coincides with the Direction of 

 the strata. 



The phenomena of metalliferous veins may be referred to, as another 

 large class of facts which demand the notice of the geologist. It 

 would be difficult to point out briefly any general laws which prevail in 

 such cases ; but in order to show the curious and complex nature of 

 the facts, it may be sufficient to refer to the description of the metallic 

 veins of Cornwall by Mr. Carne ;' in which the author maintains that 

 their various contents, and the manner in which they cut across, and 

 stop, or shift, each other, leads naturally to the assumption of veins 01 

 no less than six or eight different ages in one kind of rock. 



Again, as important characters belonging to the physical history of 

 the earth, and therefore to geology, we may notice all the general laws 

 which refer to its temperature; both the laws of climate, as deter- 

 mined by the isothermal lines, which Humboldt has drawn, by the aid 

 of very numerous observations made in all parts of the world ; and 

 also those still more curious facts, of the increase of temperature which 

 takes place as we descend in the solid mass. The latter circumstance, 

 after being for a while rejected as a fable, or explained away as an 

 accident, is now generally acknowledged to be the true state of things 

 in many distant parts of the globe, and probably in all. 



Again, to turn to cases of another kind : some writers have endeavored 

 to state in a general manner laws according to which the members of 

 the geological series succeed each other ; ind to reduce apparent ano- 

 malies to order of a wider kind. Among those who have written with 

 such views, we may notice Alexander von Humboldt, always, and in all 

 sciences, foremost in the race of generalization. In his attempt to 

 extend the doctrine of geological equivalents from the rocks of Europe 3 

 to those of the Andes, he has marked by appropriate terms the general 

 modes of geological succession. " I have insisted," he says, 3 " princi- 

 pally upon the phenomena of alternation, oscillation, and local 

 siippression, and on those presented by the passages of formations from 

 one to another, by the effect of an interior developement" 



The phenomena of alternation to which M. de Humboldt here refers 

 are, in fact, very curious : as exhibiting a mode in which the transitions 

 from one formation to another may become gradual and insensible, 



1 Transactions of the Geol. Soc. of Cornwall, vol. ii. 



* Gissfment des Roches dans les dfux Hemispheres, 1823. 3 Pref. p. vi. 



