Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/382

 of matter by degrees forms a plain or a delta at its mouth, with a level surface; the constant action of the waves continuing to level the accumulated materials until the growth of peat or soil from the ordinary decomposition of vegetables, increases its elevation to such a degree, as to exclude the further access of the waters. Instances of such deltas are too common to need mention, and they are found at the entrances of all rivers whether lateral or direct; their extent and form being proportioned to the various conditions under which the different streams have entered, and which must already be too obvious to require enumeration. If a lake under such circumstances were drained, these deltas would be found to terminate in gradual slopes, varying according to the inclination on which they were deposited.

Let us now compare such deltas with the present terraces of Glen Roy; that part of its structure which appears at first sight unconnected with the previous existence and actions of a lake. Assuming the imagined lake to have stood at the level of the lowest line, or its third elevation, it is easy to understand that the present terraces which exist at and near that level at the upper extremity of the valley, are the deltas which appertained to the Roy; while the lateral ones, wherever they are found, are the remains of similar deposits accompanying the lateral streams that still exist. To explain the present abrupt declivities of these terraces we must have recourse to those actions which have succeeded to the complete drainage of the valley, and which are precisely those already noticed in a former argument, where a shifting river wanders over an alluvial strath. In consequence of the present passage of the Roy, these deltas have been worn down at their bases, by a succession of changes of which the marks are every where seen in the minor terraces already described, that accompany its course along the valley. Hence