THE LUNAR WORLD. 91 



tending to wear down and reduce the asperities on its 

 surface. Water in its varied forms is constantly acting 

 in this way ; the rains trickle down the slopes of the 

 mountains wearing down the materials and gradually 

 tending to smooth away irregularities. Frost is also a 

 potent disintegrator of rocks. It finds its way into their 

 crevices and with irresistible power the rocks are riven 

 asunder by the expansion which the water undergoes when 

 it passes into the form of ice ; thus great blocks of rock 

 are loosened from their sites, and the other agents effect 

 their complete disruption. The inveterate action of streams 

 and rivers is gradually transforming the appearance of 

 our globe, and in the course of ages a stream will cut a 

 deep valley through the hardest rocks. In fact, there is 

 scarcely a spot on our globe in which the features of the 

 landscape have not been largely affected by water and 

 weather in some of the numerous ways in which they 

 operate. Of course all such agents are absent from the 

 moon, and hence it is impossible for us to see on our*own 

 earth any tracts of country really resembling the lunar 

 surface. 



It is true that we have volcanic districts and we have 

 rainless districts, but we should require a district not only 

 entirely volcanic, not only entirely rainless, but even devoid 

 of air itself, to reproduce the phenomena that we find in 

 the moon. There are no doubt some localities on our 

 globe which seem to suggest the mode in which some of 

 the characteristic lunar features may have arisen. We 

 have, for instance, in the Sandwich Islands the great 

 crater of Kilauea : at this wonderful spot the traveller 

 will see a large basin of molten lava surrounded by a range 



