402 Captain F. TT. Hutton on the Phenomena of the 



means. I propose, therefore, to discuss, in as much detail as 

 the length of a paper will allow^ the bearings of this theory, and 

 hope to show that it is adequate to account for all, or nearly all, 

 the phenomena observed on the globe. 



It will not be necessary to inquire into the much-disputed 

 question as to the state of the interior of the earth; for whether 

 it be solid or fluid is immaterial to the theory, provided that at 

 no considerable depth the temperature is above the melting- 

 point of ordinary rocks under the pressure of the atmosphere. 

 This is universally admitted, and is proved by the small mean 

 density of the globe and the rapid rate of increase of tempera- 

 ture as we descend into it. If^ therefore, at some distance below 

 the surface the rocks are heated above their melting-point, it 

 follows that the particles composing them must be in a state of 

 repulsion, and only kept in their position by pressure ; and if 

 that pressure is lessened at one point, the rocks will expand and 

 the diminution of pressure will be distributed through the super- 

 heated mass until it becomes again equalized throughout. In 

 like manner, it necessarily follows that if the pressure be in- 

 creased at any point, that increase of pressure will also be dis- 

 tn buted until the whole attains equilibrium once more. As we 

 know that the interior heat of the earth increases at the rate of 

 1^ F. for about 50 feet''^ in depth, it follows that this state of 

 superheating will commence at a depth of about 23 miles from 

 the surface, while above this the cohesive force will be greater 

 than the repulsion caused by heat, and the rocks therefore will 

 be more or less rigid, thus forming a rigid crust round a super- 

 heated or expansible interior. If, now, we suppose the interior 

 portion to be removed, it is evident, from what we know of the 

 strength of the materials forming the crust, that this outer por- 

 tion could not support itself, but would be crushed by the enor- 

 mous lateral thrust of the arch and would fall in. From these 

 considerations it follows that the surface of the earth is main- 

 tained in its position by three forces — viz. (1) its own weight, 

 (2) the support of the interior mass, and (3) the lateral thrust 

 of the various portions against one another ; and so long as these 

 forces are in equilibrmm, the surface will remain immoveable; 

 but if one or other of them is sufficiently changed in intensity 

 to overcome the rigidity of the crust, movement will follow; 

 and movement in one place necessarily involves corresponding 

 movements in other places, as already explained. If, for ex- 

 ample, from some extraneous cause elevation takes place in a 

 certain district, the pressure on the underlying rocks will be 



* The rate of increase is known to vary in different districts : but we may 

 feel sure that it increases faster than the mean of our observations ; for all 

 borings tend to cool the rocks whose temperature we are observing. 



