RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 95 



lier chapter, this view is no longer tenable, for we now know that the 

 condition of matter within the earth's interior, while perhaps not 

 directly comparable to any that is known, yet has properties most 

 resembling known matter in a solid state; it is much more rigid 

 than the best tool steel. While there must be reservoirs of molten 

 rock beneath active volcanoes, it is none the less clear that they 

 are small, local, and temporary. This is shown by the compara- 

 tive study of volcanic outlets within any circumscribed district. 



It is perhaps not easy to frame a definition of a volcano, but 

 its essential part, instead of being a mountain, is rather a vent or 

 channel which opens up connection between a subsurface reservoir 

 of molten rock and the surface of the earth. An eruption occurs 

 whenever there is a rise of this material, together with more or less 

 steam and admixed gases, to the surface. Such molten rock ar- 

 riving at the surface is designated lava. The changes in pressure 

 upon this material during its elevation induce secondary phenom- 

 ena as the surface is approached, and these manifestations are 

 often most awe inspiring. While often locally destructive, the 

 geological importance of such phenomena is by reason of their 

 terrifying aspect likely to be greatly exaggerated. 



Early views concerning volcanic mountains. As already pointed 

 out, a volcano at its birth is not a mountain at all, but only, so to 

 speak, a shaft or channel of communication between the surface 

 and a subterranean reservoir of molten rock. By bringing this 

 melted rock to the surface there is built up a local elevation which 

 may be designated a mountain, except where the volume of the 

 material is so large and is spread to such distances as to produce a 

 plain (see fissure eruptions below). 



In the early history of geology it was the view of the great Ger- 

 man geologist von Buch and his friend and colleague von Hum- 

 boldt, that a volcanic mountain was produced in much the same 

 manner as is a blister upon the body. The fluids which push up 

 the cuticle in the blister were here replaced by fluid rock which 

 elevated the sedimentary rock layers at the surface into a dome or 

 mound which was open at the top the so-called crater. This 

 " elevation-crater " theory of volcanoes long held the stage in 

 geological science, although it ignored the very patent fact that 

 the layers on the flanks of volcanic cones are not of sedimentary 

 rock at all, but, on the contrary, of the volcanic materials which 



