vi.] THE SLATES ON THE ROOF. 139 



to open at some fresh point of least resistance among 

 the overlying rocks. Bat where are these vents? 

 Buried deep under successive eruptions, shifted 

 probably from their places by successive upheavings 

 and dislocations ; and if we wanted to find them we 

 should have to quarry the mountain range all over, 

 a mile deep, before we hit upon here and there a 

 tap-root of ancient lava, connecting the upper and 

 the nether worlds. There are such tap-roots, probably, 

 under each of our British mountain ranges. But 

 Snowdon, certainly, does not owe its shape to the 

 fact of one of these old fire vents being under it. It 

 owes its shape simply to the accident of some of the 

 beds toward the summit being especially hard, and 

 thus able to stand the wear and tear of sea-wave, ice, 

 and rain. Its lakes have been formed quite regardless 

 of the lie of the rocks, though not regardless of their 

 relative hardness. But what forces scooped them out 

 whether they were originally holes left in the 

 ground by earthquakes, and deepened since by rain 

 and rivers, or whether they were scooped out by ice, 

 or by any other means, is a question on which the 

 best geologists are yet undecided decided only on 

 this that craters they are not. 



As for the enormous changes which have taken 

 place in the outline of the whole of the mountains, 

 since first their strata were laid down at the bottom of 

 the sea : I shall give facts enough, before this paper 

 is done, to enable readers to judge of them for 

 themselves. 



The reader will now ask, naturally enough, how such 

 a heap of beds as I have described can take the shape 

 of mountains like Suowdon. 



