Cambridge Philosophical Society. 155 



one part will have their thicker portions upwards and the other 

 downwards. The latter will manifestly be acted on respectively by 

 elevating forces greater in proportion to their mass than the former, 

 supposing the uplifting force to act uniformly over the lower surface 

 of the mass ; and consequently after the fissures are produced by 

 the elevation and extension of the mass, the lower system of wedge 

 will be forced upwards more than the upper one, and thus the two 

 systems will be relatively displaced in such a manner as to bring 

 thicker portions of them in contact than in their original positions. 

 The two systems being thus jammed into each other, the uplifted 

 mass will be supported as an arch, provided the abutments on which 

 the extremities rest be sufficiently firm to support the pressure thus 

 thrown upon them. 



If we conceive a vertical line to be drawn downwards from any 

 point in which the plane of a fault meets the earth's surface, the 

 line will not generally coincide with the plane ; and it is found that 

 the relative displacement of the portions of the mass on opposite 

 sides of the fault is usually such, that that portion in which the 

 above vertical line lies appears most elevated. 



Mr. Phillips has established this law by numerous observations, 

 as a very general one. In considering the relative positions of the 

 two systems of wedges above-described, after their displacement, it 

 will easily be seen how simply this law is generally thus accounted 

 for. 



It thus appears, also, how great horizontal forces, and conse- 

 quently great horizontal displacements may be produced, for which 

 it would be difficult perhaps so easily to account in any other 

 manner. 



Mr. Hopkins explained the probable bearing of the above views 

 on the theory of volcanos in the following manner. The tempera- 

 ture of the earth, in accordance with actual observations at com- 

 paratively small depths, may be such at the depth of twenty or 

 thirty miles, as would under the pressure of the atmosphere fuse 

 most of the substances composing the surface of the globe. No 

 complete fusion, however, of the matter of the earth at the above 

 depth can take place, because it is shown by the observed amount 

 of precession, that the thickness of the earth's solid crust must at' 

 least be much greater than that quantity. This apparent discre- 

 pancy can only be removed, it would seem, by supposing pressure 

 an antagonist cause with reference to heat considered as the cause 

 of fluidity. This still requires the verification of experiment, but 

 here assuming it to be true, it is easily seen how the crust of the 

 earth may be generally solid to any depth, while portions of it may 

 be necessarily fluid, from the removal of the superincumbent press- 

 ure by the formation over it of an arch or a dome in the manner 

 above described. 



