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Show 40 THE PAST CONDITION than the lower-there is no doubt about that; but what does this tell us about the age of any given bed in Loch Lomond, as compared with that of any given bed in the Lake of Killarney? It is, indeed, obvious that if any two sets of deposits are separated and discontinuous, there is absolutely no means whatever given you by the nature of the deposit of saying whether one is much younger or older than the other; but you may say, as many have said and think, that the case is very much altered if the beds which we are comparing are continuous. Suppose two beds of mud hardened into rock,-A and B are seen in section. (Fig. 5.) Well, you say, it is admitted that the lowermost bed is always the older. Very well; B, there fore, is older than A. No doubt, as a whole, it A B ---- i <JG. 5. is so; or if any parts of the two beds which are in the same vertical line are compared, it is so. But suppose you take what seems a very natural step further, and say that the part a of the bed A is younger than the part b of the bed B. Is this sound reasoning? If you find any record of changes taking place at b, did they occur before any events which took place while a was being deposited? It looks all OF ORGANIC NATURE. 41 very plain sailing, indeed, to say that they did; and yet there is no proof of anything of the kind. As the former Director of this Institution, Sir H. De la Beebe, long ago showed, this reasoning may involve an entire fallacy. It is extremely possible that a may have been deposited ages before b. It is very easy to understand how that can be. To return to Fig. 4; when A and B were deposited, they were substantially contemporaneous; A being simply the finer deposit, and B the coarser of the same detritus or waste of land. Now suppose that that sea-bottom goes down (as shown in Fig. 4), so that the first deposit is carried no farther than a, forming the bed At, and the coarse no farther than b, forming the bed Bl, the result will be the formation of two continuous beds, one of fine sediment (A A 1) over-lapping another of coarse sediment (B B1). Now suppose the whole sea-bottom is raised up, and a section exposed about the point A1 ; no doubt, at this spot, the upper bed is younger than the lower. But we should obviously greatly err if we concluded that the mass of the upper bed at A was younger than the lower bed at B; for we have just seen that they are contemporaneous deposits. Still more should we be in error if we supposed the upper bed at A to be younger than the continuation of the lower bed at B1 ; for A was deposited long before B1• In fine, if, instead of comparing immediately adjacent parts of two beds, one of which lies upon another, we compare distant parts, it is quite possible that the upper may be any number of years older than the under_, and the under any number of years younger than the upper. Now you must not suppose that I put this before |