PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. * 637 



without Jin accompanying volcanic eruption, the movement is the 

 same as when with ; the upward pressure being sufficient in the 

 latter case to find a vent and in the former not, and that those 

 earthquakes which are unaccompanied with volcanic eruption, ave 

 probably produced by the same movement. 



The similarity between lavas and plutonic rocks in chemical 

 composition, is a very strong indication of their production from 

 the same musses of matter; and the diiference in their specific 

 gravities is explained by the fact, that the lavas cool quickly under 

 the pressure of the atmosphere only, Avhile the plutonic rocks cool 

 slowly under the pressure of overlying strata. 



Since, according to this hypothesis, plutonic rocks are only 

 formed under the pressure of overlying strata, their exposure on 

 the surface is always due, either to a second push from beneath, 

 or to denudation. 



Whether the subterranean masses upon which these phenomena 

 depend will become exhausted, or whether new masses of molten 

 matter are forming, are questions Avhich cannot now be answered. 



If the depressions of some portions of the earth is squeezing out 

 these lakes or seas, the upheaval of other districts may be producing 

 them, and thus when in course of time the district now upheaving 

 will be depressed, it will in time become volcanic. 



What causes the upheaval and depression of the earth's surface, 

 and how far down these movements extend, are questions of great 

 importance. Since the daily revolution of the earth on its axis 

 leads to the movement of the tides, and its annual revolution 

 round the sun changes the movement of the ocean currents, may 

 not its movement with the whole solar system round a central orb 

 produce this upheaval and depression ? 



The oblation of the earth at its poles has very inconsiderately 

 been offered as a proof that the earth was once a liquid mass. 

 AVhy must we first melt the earth to tlatten it at its poles ? Why 

 not suppose it once a cylinder instead of a sphere, which rotation 

 made an oblate spheroid ? 



As Lyell well remarks, the earth has as long as we have known 

 anything about it, had the same flattening at its poles it has now. 

 Why has it not always, and without of necessity ever having been 

 liquid? In fact, the only form in which a revolving body, solid 

 or liquid, can exist in a state of stable equilibrium, is that of an 

 oblate spheroid. 



To recapitulate : 1st. The solar system has never been a neba- 



