Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/194

 Fountain Lake. It rests everywhere directly on the Eocene sands and clays ; and its lower beds are in a great measure made up of these underlying sands and clays re-deposited. This is so much the case, that it is often difficult, even in the open cuttings, to distinguish between the top of the Eocene and the bottom of the mud.

Change of level, or possibly the silting up of some narrow inlet to the tidal water, must at some time have converted this mud- deposit (A) into a land-surface, as its surface, where not eroded, is seen dotted over with the stumps of trees of small growth — possibly Alder or Willow, their roots often penetrating downwards vertically to the depth of 5 feet. The present surface of this root- bed stands at from 3 feet above to 2 feet beneath the present low- water level of the harbour. It is this root-bed, probably, which has been described by Sir Henry James * as occurring beneath the dockyard at a depth of from 4 to 14 feet beneath low water.

I do not see exactly in what way to account for the difference of level of the root-bed shown in the Mud-section and again beneath the dockyard, except by supposing a subsidence of the underlying Tertiary deposits. It is at least worthy of notice that the dip of the root-bed in this area corresponds in direction with the dip of the Eocene strata on which it rests.

The mud-bed B, or recent mud-bed as it might be called, as compared with the root-bed, commences at the foot of the low escarpment, and, spreading northwards and westwards, rests everywhere directly on the Eocene, or on the mud-bed A where this is present. The surface of the mud (B) stands at from 6 to 7 feet above low- water level, or just midway between ordinary high and low water. One may suppose, indeed, other conditions remaining the same, that this level would be indefinitely maintained, the flow, and consequently the carrying powers of the water on and off the mud being equal. A study of the sections tends to confirm this idea ; for there is evidence in these of very slow deposition near the surface of the mud in most places, and of very rapid deposition in a few others — the rapid deposition, as along the edge of Fountain Lake, being clearly the result of silting up to a certain level.

The spread or overlap of the mud-bed B so far beyond that of A seems to point to a cutting back of the gravel-capped escarpment, for the same distance, since the submersion of the root-bed.

The gravel-bed x contains no fossils.

The root-bed contains rarely a few specimens of Littorina.

The mud-bed A is crowded with recent shells at and near the surface, and usually also near its base. Antlers of the Bed and Fallow Deer have also been met with in this bed.

Thin beds of fine subangular shingle are seen in places, either interstratified in the mud or near its base, and in all cases include recent shells.

It has been my good fortune while studying the above sections to enjoy the friendship of several of the gentlemen in command of the works; and to their assistance I am indebted for much valuable


 * Quart. Journ. Geol. Soc. vol. iii. p. 249.