Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/373

 will be found level, and they assume an artificial appearance. In other cases their surfaces are irregular, and often, where the river encroaches near on the original hill, are gradually blended with it until they are lost in the general slope; the river forming a second and more abrupt one in the covering of alluvium which time or more active causes have accumulated on its face.

This sketch is sufficient to give a general idea of the nature of the hypothesis I am now about to examine.

Since the opposite lines of Glen Roy correspond at three several stages, it is apparent that the action of the water must have consisted in cutting its way through an alluvial plain, first from the highest to the lowest of these stages, and ultimately to the present bottom of the valley, or rather to the bottoms of all the different valleys which now exhibit these appearances.

It is easy to see that no set of partial alluvia, occupying the sides of hills or the entrances of lateral torrents, could have answered the necessary conditions; as in no other case than the one previously supposed, could the river have occupied the requisite elevation. When therefore it is said that the lines of Glen Roy are the remains of water terraces, it ought also to be shown how these terraces were first formed. It is evident that the circumstances here assumed are the only ones capable of terminating in the present appearances.

It is a remarkable circumstance, on any supposition, that the lines should not only be so generally equal in breadth compared with each other, but that they should be so equal throughout such a variety of ground. But on the supposition that they are the remains of terraces, and that these terraces are the relics of a prior terreplein which has been subsequently removed by the action of water, it maybe considered as impossible. The variety of ground over which they are extended, and the unequal action which water must have exerted