8 THE OEIGIN AND ALTERATION OF ROCKS. 



vents, even if they were near one another. The viscidity of the cooling 

 Hquid portion would, of itself, prevent any rapid flow of material from one 

 point to another. But at the same time the liquidity of the interior mass 

 would cause it to seek escape from pressure at any available opening, how- 

 ever far that vent might be from the point of pressure. Yet the more 

 viscous the material, the less applicable would be the ordinary law of the 

 transmission of pressures by liquids. 



The part played by water in a volcanic eruption seems to consist mainly 

 of its action on the lava during its passage upwards, instead of serving as 

 the cause or primiim mobile of the eruption. It is difficult to see how lava 

 in ascending to the earth's surface could reach it without meeting water 

 somewhere on its way. This water with its attendant phenomena seems to 

 be the accident, rather than the cause of the eruption. As stated before, a 

 different view of the present structure of the earth's interior can be taken, 

 which may not be inconsistent with the facts of petrography. This is that 

 the interior, or at least the portion from which our eruptive rocks come, is 

 solid, but in such a state that it can be readily reliquefied. This reliquefac- 

 tion may be brought about either from increase or diminution of pressure, 

 according as future experiments may show the relative densities of hot 

 solid and liquid matter to be. The supposition that eruptive rocks come 

 from these re-fused portions of the earth's originally solidified primitive 

 material, would perhaps explain the origin of the minerals of the first or 

 foreign class, to be spoken of later, which occur in these rocks. 



Section IT. — 71ie Origin and Alteration of Bocks. 



The theory of the origin of rocks generally taught in America is the 

 following, with some more or less important modifications : The sedimentary 

 (chemical and mechanical) rocks derived from the ruins of the " primeval 

 crust " form all that portion of the earth's crust which is now known. By 

 ordinary denudation these rocks would be removed from one point and depo- 

 sited in another locality, the result being that the underlying sediments would 

 be still more deeply buried in one place, and exhumed in another. The por- 

 tions thus more deeply buried would be invaded b}^ the earth's central heat, 

 this giving rise to a more or less intense chemical action in them. The seat 

 of this action is known as the " zone of aqueo-igneous fusion " (solution), and 

 all sediments, if sufficiently deeply buried, come within this hypothetical 



