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84 the sands. One of these, just outside the Hamoaze, or estuary of the Tamar, shows a rocky bottom at 150 feet, probably the true rock-floor of that part of the Sound. The other hollow, 132 feet, is just outside the Cattewater; but does not reach rock.

It is obvious that these depths, both of which are measured from low water, show a depression of the rocky floor of the Sound far greater than we meet with in ordinary Pleistocene valleys; but at present we have no means of proving the true date of this depression. It represents not improbably a Tertiary basin, like that of Bovey Tracey, which also descends several hundred feet below sea-level. In favour of this view we can point to the occurrence of a small outlier of Trias in Cawsand Bay, which certainly suggests that the Sound represents an area of depression or synclinal basin, rather than a mere submerged valley. It has also been stated that relics of Tertiary material are still to be found in the limestone quarries of Plymouth; but for this the evidence is not altogether satisfactory.

It may be asked, What practical difference does it make, whether or no the Plymouth Sound were originally a Tertiary basin, for no Tertiary gulf could now remain open? If we were dealing with an area of soft rocks, like the Thames Valley, or with an enclosed sea, this objection would hold. Around Plymouth, however, the Palaeozoic rocks are