308 GENERAL COMPARISON, &C. 



in every respect perfect. This fact is sufficient to prove 

 that neither elevation nor subsidence have been the effi- 

 cient causes of the discontinuities in question, and that 

 they have not been the result of any sudden and violent 

 action. To a certain extent at least, there seem reasons 

 for supposing, that they have been produced by a more 

 gradual power, and we may perhaps, by combining both 

 hypotheses, arrive at an explanation not very improbable. 

 It is conceivable that the same change which gave the 

 whole line of strata its present position, was accompanied 

 by fractures across its direction in the places now occupied 

 by these straits. Thus the geological continuity might 

 remain, as now, unbroken ; while the action of well known 

 causes operating through a long period, would tend to 

 enlarge the original openings to their present condition. 

 Of this gradual enlargement there is visible proof in the 

 present wearing of the strata on the sides of the straits. 

 In the sound of Isla it is obvious to the most superficial 

 observation, and is registered in a most impressive manner 

 by the permanence of those trap veins which still remain 

 standing alone on the shores like walls ; monuments of 

 the destruction of the softer strata. 



