250 TROPICAL NATURE, AND OTHER ESSAYS. 



furnish matter for a few observations, of some interest 

 to biologists and at the same time not unintelligible to 

 the less scientific members of the Association who may 

 honour us with their presence. 



The subjects I first propose to consider have no general 

 name, and are not easily grouped under a single descrip- 

 tive heading; but they may be compared with that 

 recent development of a sister science which has been 

 termed surface-geology or Earth-sculpture. In the older 

 geological works we learnt much about strata, and rocks, 

 and fossils, their superposition, contortions, chemical con- 

 stitution, and affinities, with some general notions of 

 how they were formed in the remote past ; but we often 

 came to the end of the volume no whit the wiser as to 

 how and why the surface of the earth came to be so 

 wonderfully and beautifully diversified ; we were not 

 told why some mountains are rounded and others pre- 

 cipitous ; why some valleys are wide and open, others 

 narrow and rocky ; why rivers so often pierce through 

 mountain-chains ; why mountain-lakes are often so 

 enormously deep ; whence came the gravel, and drift, 

 and erratic blocks so strangely spread over wide areas 

 while totally absent from other areas equally extensive. 

 So long as these questions were almost ignored, geology 

 could hardly claim to be a complete science, because, 

 while professing to explain how the crust of the earth 

 came to be what it is, it gave no intelligible account of 

 many phenomena presented by its surface. But of late 

 years these surface-phenomena have been assiduously 

 studied ; the marvellous effects of denudation and glacial 

 action in giving the final touches to the actual contour 

 of the earth's surface, and their relation to climatic 



