Chap. XVIII.] THE SURFACE OP THE EAETH. 201 



overlapped that to the west. So that on the west, 

 the Pacific Ocean rolls against the disjointed masses 

 that have been piled up about it ; whereas on the 

 east, the elevated strata slope away gradually to the 

 Atlantic Ocean. That slope is itself undulated by 

 pressure ; but those undulations are probably prece- 

 dent to the occurrence of the crack which led to the 

 piling up of the Andes ; most, if not all, subsequent 

 readjustments of the surface having been arranged 

 by movements along the still unfirmly placed edges 

 of the crack. The sudden movements in this neigh- 

 bourhood even now cause at times a shock, or earth- 

 quake, sufficient to overwhelm cities. In these 

 movements, also, either by direct pressure of the 

 surface downwards, or oftener probably by water or 

 other matter being suddenly brought into contact 

 with intense heat, matter from below the stratified 

 surface is, in a state of liquid heat, forced upwards 

 through openings in the crack ; thus forming, as the 

 matter hardens on the surface, those high volcanic 

 peaks which are here so numerous. Or, in other 

 places, the same expansion not having sufficient 

 force to burst through the surface, simply raises it in 

 the form of an evenly rounded hill. 



Now, if these observations be not erroneous, we 

 must conclude that granite, and most unstratified 

 rocks, are in general forced to the surface in the same 

 manner as is mud through the fissures between 

 paving- stones ; that is, not by the direct pressure of 



