ON THE LEVEL OF THE SEA. 81 



be made. I am convinced, however, that the dispropor- 

 tion will be found to be no less than the one above stated ; 

 and that by a careful examination of these dykes, which 

 afford, as Dr MacCulloch remarks,* the most perfect re- 

 cord which geology furnishes of the wasting action of the 

 sea, we may even attain something like an estimate of 

 the actual time which has elapsed during their formation. 

 Could we ascertain the rate of the wasting action of the sea, 

 and the extent of rock removed by it, we should have the 

 answer at once. I do not despair of arriving at a proxi- 

 mate value of both these elements of the calculation. In 

 the example I have given, we have ocular evidence that the 

 sea, when it stood at its former level, must have removed 

 upwards of 400 feet of the sandstone rock ; to this must 

 be added the amount of what has been wasted since the sea 

 stood at its present level. This, however, cannot be much, 

 judging from the relative dimensions of the outstanding 

 dykes. It will be seen that the waste of sandstone on the 

 remaining part of the platform is not quite double that of 

 the trap ; if we assume that it has been a little more, and 

 take 420 feet as the amount removed, at the same rate we 

 have 26^ feet the amount removed by the sea at the pre- 

 sent level. This agrees very well with the shape of the 

 ground, and is probably not far from the truth. With re- 

 gard to the rate at which the sandstone of C umbra yields 

 to the wasting process which is continually acting, all I 

 can say is, that it is too slow to be perceptible in the life- 

 time of one generation. I can appeal to my own recollec- 

 tions of these rocks for nearly half a century, and, except 

 in one instance, can observe no change in their form. The 

 one I allude to, is vividly impressed on my memory as 



* MacCulloch's Western Islands. 

 VOL. VIII. F 



