vi.] THE SLATES ON THE ROOF. 147 



Rogers, again, points out that as these slates may have 

 been highly heated, thermal electricity (i.e. electricity 

 brought out by heat) may have acted on them. 



One thing at least is clear. That the best slates 

 are found among ancient lavas, and also in rocks which 

 are faulted and tilted enormously, all which could not 

 have happened without a proportionately enormous 

 pressure, and therefore heat ; and next, that the best 

 slates are invariably found in the oldest beds that is, 

 in the beds which have had most time to endure the 

 changes, whether mechanical or chemical, which have 

 made the earth's surface what we see it now. 



Another startling fact the section of Snowdonia, 

 and I believe of most mountain chains in these islands, 

 would prove namely, that the contour of the earth's 

 surface, as we see it now, depends very little, certainly 

 in mountains composed of these elder rocks upon the 

 lie of the strata, or beds, but has been carved out by 

 great forces, long after those beds were not only laid 

 down and hardened, but faulted and tilted on end. 

 Snowdon itself is so remarkable an instance of this 

 fact that, as it is a mountain which every one in these 

 happy days of excursion-trains and steamers either 

 has seen or can see, I must say a few more words 

 about it. 



Any one who saw that noble peak leaping high 

 into the air, dominating all the country round, at least 

 upon three sides, and was told that its summit con- 

 sisted of beds much newer, not much older, than the 

 slate-beds fifteen hundred feet down on its north- 

 western flank any one, I say, would have the right at 

 first sight, on hearing of earthquake faults and up- 

 heavals, to say The peak of Snowdon has' been 



L 2 



