OUR MOLTEN GLOBE 55 



suffices to keep its lava in permanent ebullition ; while 

 the lofty Mauna Loa has its vent usually blocked up, and 

 may owe its occasional eruptions to an accumulation of 

 gases in some deep-seated cavities which, at long intervals, 

 become sufficiently powerful to burst away the obstacle 

 and pour out a quantity of melted material derived from 

 the sides of the channels through which they make their 

 way upward. 



The phenomena presented by the crater of Kilauea, 

 where an extensive lava-lake remains in a constant state 

 of ebullition while keeping approximately the same level, 

 can only be explained by the upward j^ercolation of heated 

 gases in moderate and tolerably uniform streams, suffi- 

 cient to keep up the melting temperature of the lava ; 

 while occasional more powerful outbursts throw up jets or 

 waves of the molten matter, or sometimes break up the 

 crust that has formed over portions of the lake. Here, 

 evidently, there is no eruption in the ordinary sense, no 

 fresh matter is being brought up from below, but only 

 fresh supplies of intensely heated gases sufficient to keep 

 the lava permanently liquid, and to produce the jets, 

 waves, and fountains of lava, and the strange surging, 

 swirling, and wallowing motions of the molten mass, so 

 well described by Miss Bird, Lord George Campbell, and 

 other competent observers. 



The sketch now given of Mr. Fisher's investigations as 

 to the nature of the molten interior of the earth and of 

 the crust which overlays it, only covers a small portion of 

 the ground traversed in his work. He there deals also 

 with the more difficult questions of the stresses produced 

 by the contraction of the cooling earth, and the various 

 theories that have been suggested to explain the great 

 inequalities of its surface. The origin of the great oceanic 

 depressions and of the vast mountain masses that every- 

 where diversify the continental areas, and the causes that 

 have produced the compression, upheaval, folding and 

 crumpling of the rocks at every period of geological 

 history, are all discussed, and some light is thrown upon 

 these confessedly obscure and very difficult problems. 



