513 



by a reference to existing causes, are his philosophical views of the 

 relations of climate to the former extension of land and water, which 

 have resulted in doctrines of the highest importance. This prin- 

 ciple, long disputed, is now so far recognized, that no cautious 

 naturalist declares himself satisfied with the explanation of a geo- 

 logical phenomenon except he can test or illustrate it by agencies 

 now in operation. It is true that Hutton and Playfair originated 

 the hypothesis that causes such as are now in action would account 

 for all we see on the surface of the globe ; but we owe to Sir 

 Charles Lyell that careful collection of facts and that sagacious 

 reasoning, by which this hypothesis has been raised to the rank of a 

 well-grounded theory. 



We cannot overrate the services which he has rendered by his in- 

 vestigations into the dynamics of geology ; nor the importance of his 

 memoirs on the risings and subsidences of land, and their incessant 

 fluctuations, on the means, extent, and effects of the distribution 

 of sediments, and the changes wrought on these by metamorphic 

 action, and on the amount of denudation which all formations have 

 undergone. These, and a vast number of subsidiary phenomena, 

 have led him by an elaborate train of reasoning, to gauge boldly the 

 depth of detrition to which our planet has been subjected, by the 

 miles of sedimentary rocks accumulated on its surface; and to those 

 startling speculations on the antiquity of the earth's crust, which, 

 having survived the ridicule and contempt of many critics, and 

 the rejection by more as visionary, are now becoming accepted as 

 fundamental truths. 



The geology of volcanos in its widest sense presents another great 

 field for observation and reflection, to which Sir C. Lyell has assidu- 

 ously applied himself ; and here I can do no more than allude to 

 his discoveries and discussions on "craters of elevation," the amount 

 of denudation which volcanic cones have sustained, their valleys 

 and dykes, and the inclination of their lava beds, as all of the 

 greatest importance ; especially, however, referring to the results 

 of his recent studies of Etna, because they afford the clearest 

 demonstration of the genesis and internal structure of volcanic 

 mountains. 



There is indeed no department of geology, from the Silurian 

 strata to the mud now depositing on our coasts and in our lakes 



