494 



Recent Literature. 



also the internal structure of volcanic mountains and their distri- 

 bution over the earth. 



The nature of lava overflows and the causes of the differences 

 in their rate of motion are well discussed. The accompanying 

 illustration (Fig. 2) of a lava stream which, from its imperfect 

 fluidity in flowing over the edge of a precipice, forms heavy pendant 

 masses like a " guttering " candle, is a fair example of the qual- 

 ity of the woodcuts. 



The volcano from which all our pumice-stone comes is rep- 

 resented by Fig. 3. How this is formed is well told. By experi- 

 ments with sawdust thrown up by an air-blast, the structure of 

 volcanoes formed of scoriae, pumice and other fragmental ma- 

 terials is illustrated. 



" Many cones formed in the first instance of scorise, tuff and 



pumice may give rise to streams of lava, before the ven 

 they surround sinks into a state of quiescence. In these cases, 

 the liquid lava in the vent gives off such quantities of steam that 

 masses of froth, or scoriae, are formed, which are ejected, and 

 accumulate around the orifice. When the force of the explosive 

 action is exhausted, the lava rises bodily in the crater, which it 

 more or less completely fills. But eventually the weaker side ot 

 the crater wall yields beneath the pressure of the liquid mass, 

 and this part of the crater and cone is swept away before the 

 advancing lava stream. Examples of such 'breached cones 

 abound in Auvergne and many other volcanic districts. A beau- 

 tiful example of a cone formed of pumice, which has been 

 breached by the outflow of a lava stream of obsidian, occurs in 



