It is necessary to make this digression as to the effects of tidal scour, for we are sometimes told that the various basins, troughs, and channels shown on the charts represent submerged land-valleys, and thus prove enormous changes of sea-level in modern times. How a submerged valley in a narrow sea with sandy bottom, like the English Channel, could remain long without silting up is not clear; the sand-banks on either side should tend to wash into and fill up the hollows. The troughs, however, all coincide with lines of tidal scour; they do not continue the lines of existing valleys, unless these valleys are so large as to produce a great scour, and unless this scour is aided by the oscillation of the waves. A glance at the Admiralty chart will show that no submerged channel crosses the direction of the tidal scour or of the Atlantic swell; the channels are scoured where tide and swell act together. We conclude therefore that Plymouth Sound probably represents a basin once filled with soft Tertiary and Secondary deposits, and that these soft deposits were cleared out by the sea, leaving the rocky floor of the basin bare at a considerable depth below sea-level. In part the basin has now silted up again; but we may fairly consider that at the time of greatest elevation, when the submerged valleys were being eroded, the depth of water in the Sound was much the same as it is now. Then as now the rivers seem to have discharged into a wide open gulf occupied by the sea. However this may be, we see now a series of deeply trenched valleys, partly submerged and all opening into a wide and deep bay. These valleys do not now show rocky bottoms gradually sloping into the open harbour. The rock floor ceases several miles up and gives place first to an alluvial flat and then to an arm of the harbour. Like all the other valleys with which we have been dealing they cut to a definite base-level, approximately that of the sea, and the parts below that level are rapidly silting up. Fortunately a large series of bridge-foundations has shown well the character of these valleys, where the rocky floor passes beneath the sea-level, and the late R. H. Worth gave an excellent series of sections across them. He took their contours to be evidence of glaciations. In this I cannot agree with him; but think rather that the extraordinary flatness of the valley-bottoms, and especially the uniform depth to which they were excavated, point to the attainment of a definite base-level.