138 TOWN GEOLOGY. [vi. 



And therefore I beg my readers to put out of their 

 minds once and for all the fancy that in any known 

 part of these islands craters are to be still seen, such 

 as exist in Etna, or Vesuvius, or other volcanoes now 

 at work in the open air. 



It is necessary to insist on this, because many 

 people hearing that certain mountains are volcanic, 

 conclude and very naturally and harmlessly that the 

 circular lakes about their tops are true craters. I have 

 been told, for instance, that that wonderful little blue 

 Glas Llyn, under the highest cliff of Snowdon, is the 

 old crater of the mountain ; and I have heard people 

 insist that a similar lake, of almost equal grandeur, in 

 the south side of Cader Idris, is a crater likewise. 



But the fact is not so. Any one acquainted with 

 recent craters would see at once that Glas Llyn is not 

 an ancient one ; and I am not surprised to find the 

 Government geologists declaring that the Llyn on 

 Cader Idris is not one either. The fact is, that the 

 crater, or rather the place where the crater has been, 

 in ancient volcanoes of this kind, is probably now 

 covered by one of the innumerable bosses of lava. 



For, as an eruption ceases, the melted lava cools in 

 the vents, and hardens; usually into lava infinitely 

 harder than the ash-cone round it ; and this, when 

 the ash-cone is washed off, remains as the highest 

 part of the hill, as in the Mont Dore and the Cantal 

 in France, and in several extinct volcanoes in the 

 Antilles. Of course the lava must have been poured 

 out, and the ashes blown out from some vents or 

 other, connected with the nether world of fire ; 

 probably from many successive vents. For in 

 volcanoes, when one vent is choked, another is wont 



