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I will only touch upon one other instance of the irregularity of deposition 

 such as might occur in Lacustrine formation. Suppose, for example, a chain 

 of lakes in some mountainous district connected together by a river flowing 

 in at the one end and out at the other. During countless ages this river 

 or stream may have been bringing down a regular deposit of sand, gravel, 

 or stones from one of the upper lakes to one of the lower ; but suppose 

 from some of the many causes which affect the levels of the surface of our 

 earth, an unusual flood should occur and the barrier of one of the lakes be 

 swept away : the result would be that a great portion of the water of the 

 lake would be, in a very brief period of time, precipitated down the valley into the 

 next lake, and, bearing in mind the enormously increased power of water in rapid 

 motion, it is easy to understand how such a catastrophe would in a few hours 

 destroy the quiet work of deposition which might have been going on for ages, 

 and replace it by a vast deposit of heavier materials. In this case — which is a 

 very common one — the record of ages may be swept away in a few hours. 



In a short paper like the present it will not do to spend too much time over 

 any one particular branch of the subject, so I must now pass on to consider how our 

 estimate of geological time may be affected by the work of denudation. Denuda- 

 tion, as no doubt all who hear me are aware, is the opposite process to deposition, 

 and is the wearing away either by the action of the atmosphere, or water, or ice, 

 of some portion of the earth's surface. This action may be said to be going on 

 more or less all over the portion of this earth which is above the sea level ; but I 

 need hardly point out how much greater are the visible effects of denudation by 

 the sea coasts, and by the banks of rivers and glaciers — which latter may be 

 compared to frozen rivers. Now, to turn to our particular subject of measuring 

 the time of geological action, it has constantly been observed that at certain parts 

 of the coast the land is gradually receding, and upon some measured data of this 

 kind, conclusions are drawn as to the length of time that the sea has been making 

 such a channel, or the length of time which will be required to remove the coast 

 so much farther inland, and so forth ; and no doubt these conclusions are logical 

 just so far as we are able to trust the premises. In many cases the action of the 

 sea appears to be very regular, and in these cases our inferences are, no doubt, 

 correct, — or nearly so— but there are a far more numerous class of cases where, on 

 account of inequality of hardness of rock, the action of the sea upon the coast is 

 very irregular. This, of course, is one of the chief causes of the indentations of 

 the coast line into bays and capes. Here, then, it is evident that the action of 

 the sea upon a soft and yielding rock is very much greater than upon a hard one ; 

 and as, when once the rock is removed, we have no evidence left of what sort of 

 rock it was — and hence no evidence of the time it took in its removal— it is 

 palpable that no trustworthy conclusions can be based upon any action we see 

 going on, unless we can also satisfy ourselves that the rocks have all been of the 

 same kind, which, without any direct positive evidence, it is very difficult to 

 believe. The process of excavation by water is well seen in the case of some of 

 the great rivers of the world, but in no instance that I know of is there a better 



