808 THE WONDERS OF GEOLOGY. Lect VIII. 



and exhalations of steam, carbonic acid gas, sulphurous 

 vapours, &c. and the production of various rocks and 

 minerals.* In the earlier ages of the globe the phenomena 

 appear to have been of greater extent and in a higher 

 degree of intensity than in modern times. 



3. Earthquakes. — The tremblings or vibrations of 

 the solid crust of the globe, denominated Earthquakes, are 

 vertical, horizontal, or gyratory oscillations of the land 

 rapidly succeeding each other; and are caused by the 

 expansive efforts of elastic fluids confined in subterraneous 

 cavities. The craters of volcanoes are the vents through 

 which the imprisoned gases and vapours, and the incan- 

 descent mineral matter, force their way to the surface : they 

 are, in fact, the safety-valves of the vast reservoirs of 

 gaseous elements which are contained in the profound 

 depths of the earth. Hence, if the action of a volcano be 

 impeded, earthquakes are commonly induced, and the 

 equilibrium of the land is not restored till the crater resumes 

 its activity, or the imprisoned gases escape through other 

 channels, f A column of smoke which was seen for some 

 months to rise from the volcano of Pasto in South America, 

 suddenly disappeared, when on the 4th of February, 1797, 

 the province of Quito, 192 miles to the southward, was 

 visited by the great earthquake of Riobamba.J 



The focus of volcanic action must be at an immense dis- 

 tance from the earth's surface, though no rational conjecture 

 can be formed either of its depth or of the chemical nature 

 of the compressed fluids : but the vast areas over which 

 the undulations sometimes extend, are proofs of the tremen- 

 dous power of these subterranean forces. 



* Humboldt's Cosmos, p. 181. 



t The explosion of a steam-boiler from the closure of the safety 

 valve (as in the recent lamentable catastrophe of the Cricket steam- 

 boat) is a familiar example of this phenomenon. 



X Cosmos, p. 203. 



