IGNEOUS AGENCIES. 



133 



canoes are openings into local reservoirs, not into a uni- 

 versal sea of liquid matter. 



Kecently there has been a tendency among geologists 

 to accept a compromise between these extremes. It is 

 now well known that rocks, under the combined influence 

 of heat and water, fuse at a much lower temperature. 

 This, to distinguish it from true dry fusion, is called 

 hydrothermal fusion. While the temperature of true 

 fusion is not less than 3,000, that of hydrothermal fusion 

 is only 600 to 800. Now, water certainly penetrates the 

 earth to great depths. Therefore many think that the 

 general constitution of the earth is that of a solid nucleus 

 and a solid crust, separated by a sub-crust layer of liquid 

 or semi-liquid matter. There are many geological phe- 

 nomena that are best explained by such a supposition. 



The interior heat of the earth is the source of all igne- 

 ous agencies. It shows itself on the surface in three 

 principal forms, viz.: 1. Volcanoes ; 2. Earthquakes; 3. 

 Gradual oscillations of the crust. 



/SECTION I. VOLCANOES. 



Definition. A volcano may be defined as a conical 

 mountain, with a pit-shaped, cup-shaped, or funnel- 

 shaped opening atop, from which are ejected, from time 

 to time, materials of various kinds, always hot and often 

 fused. They vary in size from inconspicuous mounds to 

 mountains many thousand feet high. 



Volcanoes may be active or extinct. Those which have 

 not erupted for a century past are supposed to be extinct. 

 Yet, so-called extinct volcanoes sometimes break out 

 again. Until the great eruption which destroyed Hercu- 

 laneum and Pompeii, Vesuvius was supposed to be an 

 extinct cone. Since that time it has been very active. 

 Again, in some rare cases, volcanic eruptions are constant. 

 Stromboli and Kilauea, for example, are in feeble erup- 



