302 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



rarified. If we suppose the water rushes out in a fine jet, this will 

 increase in height when the air is rarified by a few strokes of the piston, 

 and it will diminish, on the contrary, when the air is compressed. We 

 have before us exactly the image of what takes place in nature, if we 

 admit that the reservoirs of most springs are cavities i^roduced by the 

 separation of sedimentary strata, following a commotion of the crust or 

 a sinking of the inferior strata, while the upper strata remained un- 

 moved, or under other circumstances were elevated. These excavations 

 fill with water, which does not vary in volume as long as the surround- 

 ing ground is in repose. But if a slow upward movement diminishes 

 the capacity of these reservoirs, the water they contaiii will be driven 

 forth with more or less violence according to the rapidity of the upward 

 movement. An earthquake may then produce such a pressure upon the 

 waters of the reservoir that it will be driven out with great force and 

 may sometimes occasion extensive inundations. In this way may be 

 explained the inundation of the island of Lacerotte, of which we have 

 already spoken. In other cases the commotion may produce large sub- 

 terranean cavities, which would absorb the water of lakes and rivers. 



The geysers of Iceland and the torpedoes of Brazil are springs of an- 

 other origin. Their waters penetrate by fissures to the incandescent 

 rocks of the volcanic outlets. There, heated by contact with the ignited 

 masses, they dissolve large quantities of silica, and, driven by the steam 

 which is formed, they rush to the surface and there, in cooling, deposit 

 the silica. To the action of hot springs we must attribute the formation 

 of many rocks, generally considered of volcanic origin. Thus hyper- 

 sthenite has been hitherto described as an eruptive rock, but is found, 

 according to the report of the geological commission of Canada, in the 

 state of sedimentary rock in several localities in that country. The sed- 

 imentary origin of many granites, syenites, diorites, and dolorites is 

 equally established by the savans of this commission. 



We have only to say, in terminating our views of plutouian forces, 

 that the metamorphism, to which is attributed all change of aggrega- 

 tion of a rock, should be reduced to its normal proportions, for the oil- 

 beds — the saccharoid marbles — are found far from all eruptive rocks, 

 and are continued often by masses which do not bear the same charac- 

 teristics. In many cases the influence of chemical agents, of enormous 

 pressure, and of heat alter the lower strata of the crust sufficiently to 

 explain some cases of metamorphism. 



We have endeavored to show that the central heat is a constant 

 source of forces which have modified the surface of the globe and 

 brought it to its present condition. These changes were produced very 

 slowly on account of certain very simple laws, which are still in force. 

 Without the necessary connection of these forces, which renders it pos- 

 sible to submit their effects to continuous observations and to deduce 

 from them the bases of the theory of the physical constitution of the 

 earth, geology as a science could not exist; it would be undistinguish- 

 able from petrology or palsontology. upon which it is founded. 



