204 PHYSIOGRAPHY. [ch. xit. 



those into which a mass of starch splits during drying. 

 Similar evidence of volcanic action may be found in Scotland, 

 whilst in North Wales there are extensive remains of eruptive 

 rocks ; the state of fiery activity which they indicate dates 

 back, however, to a very remote period of geological history. 

 Yet it must not for a moment be supposed that any volcanic 

 crater still exists as a crater amongst the volcanic hills of 

 Wales. So great indeed has been the disturbance and de- 

 nudation of this part of the earth, that the old surface has 

 long ago been swept away, and its present shape bears litde 

 or no relation to its form at the period of eruption. It is 

 true, for example, that the summit of Snowdon is formed of 

 volcanic rocks, yet the mountain, in its present form, does 

 not in any way represent an old volcanic cone. 



Without pursuing this subject further, enough has been 

 said to prove that peaceful as our islands now are, they have 

 again and again been the scene of violent volcanic disturb- 

 ances. Fire, indeed, has played as important a part as 

 water in the geological history of the British Isles ; and it 

 is highly probable that, at a depth, which, as compared with 

 the diameter of the earth, may be justly termed insignificant, 

 even the peaceful valley of the Thames is underlaid by an 

 ocean of molten rock. 



