OF VOLCANIC ROCKS. 65 



with physical laws. Supposing for a moment that theory to be correct which consid- 

 ers all the changes of level as originating in contraction alone, then it is quite proba- 

 ble that fissures would be formed in the center of the areas of subsidence. But as 

 they must be closed near the surface, and open in their lower parts, it is difficult to 

 see in what manner the liquid matter could ascend through them to the surface, while 

 no better account could be given for the occurrence of eruptive rocks on high table- 

 lands. In order to explain it, recourse has been had to the most arbitrary assump- 

 tions. It may even be read in our time in various geological books, that the crust below 

 might give way from the overload, and the whole be " plunged " into the semi-fluid 

 mass beneath, causing it to overflow. Mountain ranges of a thousand miles in extent 

 have been assumed to subside suddenly upon the liquid mass, and to " splash it out " 

 through fissures. Leaving out of consideration these fantastical theories, there remains 

 a number of others, according to which the weight of portions of the crust would 

 cause the ejection of liquid matter. If the substance composing the crust exceeded 

 in specific gravity the liquid matter below, then this mode of ejection would be prob- 

 able, and we should be indeed surprised that large portions of our globe were not 

 flooded over repeatedly by molten masses from below. If the specific gravity of the 

 crust and the fluid matter below was the same, then it would require the most exten- 

 sive fracturing of the crust and uplifting of its fragments, in order to make the liquid 

 mass overflow the latter. But all that we know in regard to the subject goes to show 

 that the masses below the crust are of greater specific gravity than those composing 

 it. To suppose the weight of the crust to cause the protrusion of liquid matter 

 through fissures, is therefore to suppose an action which is mechanically impossible. 



We arrive at no more satisfactory conclusion in regard to our present problem, 

 if, besides the contracting forces, we assume the existence, beneath the crust of the 

 earth, of others which have their origin in the increase of volume by crystallization, 

 and thus produce expansion. I attempted to show that they furnish the most probable 

 agent to which may be ascribed the opening of the systems of fissures, which partly 

 served as the channels for the extrusion of rocky matter. These fissures would have 

 to be open at the surface and to decrease in width below, because formed by an out- 

 ward tension on areas of elevation. It cannot be assumed that they would descend to 

 any greater depth than the lowest limits of solid rocks ; they would, therefore, not 

 reach down to any matter sufficiently liquid as to be capable of being forced up 

 through them. And if it should be able to ascend, then it would solidify within the 

 fissure, almost instantly, by loss of heat, and long before reaching the surface. There 

 is, however, even a more forcible argument to demonstrate why it should not have 

 been capable of ever entering the fissures. For, if our supposition that the silicious 

 masses beneath the crust increase in volume by crystallization is correct, the relief 

 from pressure by the formation of fissures must have the immediate effect of rapidly pro- 

 experiment established by Daubree. But the supposition was only made for the case of lava, and not for that of rocks which 

 were ejected without volcanic action proper. Yet even in regard to volcanoes it did only explain the extrusion of lava to 

 the surface from a place at a limited distance below it, and failed to give a clue to the manner in which the constant supply 

 of matter to those places was kept up. 



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