Geological Origin of the Presejit Scetiery of Scotland 9 



Firth, in fact, runs along an axis of upheaval, from each side of 

 which the rocks dip away in opposite directions, like the two sides 

 of a roof Now, the old volcanic rocks, as the reader will per- 



FlG. 3. — Section across the valley of the Firth of Tay. 

 s. Lower old red sandstone ; i. Trap-rocks. 



ceive, have been bent over just as the sandstones and conglo- 

 merates have been. They do not come bursting up irregularly 

 and give rise to hills in consequence. On the contrary, they 

 have suffered from all the subterranean and superficial movements 

 which have affected the stratified rocks among which they occur. 

 They have been wholly removed from the valley of the Firth ; for 

 it is plain that the beds of trap on the one side are only the con- 

 tinuation of those on the other. Their truncated edges look 

 across at each other, and rise into those groups of terraces 

 so well marked along both sides of the Firth of Tay. Nor 

 must we fail to observe that here again there is a valley where 

 there ought, according to geological structure, to have been a Avide 

 hill. If the lower edge of the trap beds is prolonged upward at 

 the same angle as the sandstones below, it will rise into a high 

 dome. And had the shaping of our hills and valleys depended 

 chiefly upon the way in which the rocks had been folded by sub- 

 terranean operations, what is now the Firth of Tay must have been 

 a long swelling ridge, much higher than the existing hills on either 

 side of it. 



III.— The valleys are i?i the great majority of cases independent of 

 any faults or fractures of the crust of the earth. 



We have already seen that the whole of this country is more or 

 less cracked by faults ; and as it is everywhere crossed by valleys, 

 the case could hardly have been otherwise than that the valleys 

 should sometimes correspond with the line of the faults. But no 

 geologist can carefully map a large area of this country without 

 becoming convinced, even it may be in spite of a previous belief 

 to the contrary, that such examples of coincidence are quite excep- 



