RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 143 



more or less irregular form, no one dimension is of a different order 

 of magnitude from the others. Such masses are commonly de- 

 scribed as bosses, or, if especially large, as batholites (Fig. 152). 

 Wherever the rock beds appear as though they had been forced 

 up by the upward pressure of the igneous mass, the latter takes 

 the form of a mushroom and has been described as a laccolite 

 (Figs. 479-481 , pp. 441-442) . Evidence seems, however, to accumu- 

 late that in the greater number of cases the molten rock has fused 

 its way upward, in part assimilating and in part inclosing the rock 

 which it encountered. This pro- 

 cess of upward fusion has been 

 likened to the progress of a red 

 hot iron burning its way through 

 a board. 



The formation of lava reser- 

 voirs. The discarding of the 

 earlier notion that the earth has 

 a liquid interior makes it proper 

 in discussing the subject of vol- 

 canoes to at least touch upon 

 the origin of the molten rock 

 material. As already pointed 

 out, such reservoirs as exist 

 must be local and temporary, 

 or it would be difficult to see 

 how the existing condition of 

 earth rigidity could be main- 

 tained. From the rate at which rock temperatures rise, at 

 increasing depths below the surface, it is clear that all rocks would 

 be melted at very moderate depths only, if they were not kept in a 

 solid state by the prodigious loads which they sustain. Any relief 

 from this load should at once result in fusion of the rock. 



Now the restriction of active volcanoes to those zones of the 

 earth's surface within which mountains are rising, and where 

 in consequence earthquakes are felt, has furnished us at least a 

 clew to the origin of the lava. Regarded as a structure capable 

 of sustaining a load, the competency of an arch is something quite 

 remarkable, so that the arching up of strong rock formations into 

 anticlines within the upper layers of the zone of flow, or of com- 



FIG. 152. Diagram to illustrate a prob- 

 able cause of formation of lava reser- 

 voirs, and to show the connection 

 between such reservoirs and the vol- 

 canoes at the surface. 



