Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/400

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argument even against that theory which I feel most inclined to adopt. But I must now add, that the numerous difficulties which attend it have suggested to others a different mode of explaining the nature of the obstacles by which the water was maintained at the requisite height. It has been supposed that the vallies have been always the same as they are at present found, and that the imaginary dams were no other than the waters of the ocean; or in other words, that the sea itself formed the lake, to the action of which on the sides of the hills the present levels must be attributed. It will not require many words to examine the probability of this supposition, since many of the arguments already used to refute some of the hypotheses which have been examined, are equally valid against this one. Unquestionably, numerous phenomena, too well known to require notice here, justify us in believing that the waters of the sea have in former times occupied higher levels than they do at present; although we are neither able to conjecture whether these elevations were transitory or of long duration, nor to form any rational conjecture of the causes by which they were produced. In estimating the probability of this cause as applicable to the solution of the phenomena in question, it must be recollected, that the operation of the supposed lakes in producing the lines has been tedious, if we may be allowed to judge from the apparently slow operations of existing lakes in producing similar shores on their margins. It must equally be remembered, that if Glen Roy was then open to the sea, and that its lines are to be considered as ancient sea shores, the ocean must have undergone three several depressions of level at long and apparently equal intervals of time, the last of which, it must be supposed, reduced it to its present state. On considering the elevation of the uppermost line, it is plain that the ocean must in this case have covered the greatest