Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/546

450 the whole of the high-level gravel of Mr. Prestwich may be nothing more than higher terraces of the valley-gravel.

To enable the reader to understand at a glance the positions of the gravels I am about to describe, I have drawn a map of this part of the Thames valley, with contour lines of 10-feet levels strongly marked. A portion only of this map is given in the accompanying illustration (fig. 1), showing the part of the valley between Acton and the Thames. The margin of the London Clay is here shown by a dark tint, that of the gravel by a light one, and, for the sake of clearness, no distinction is made between the gravel and brick-earth. The geology of this district is taken chiefly from Mr. Mylne's map, the accuracy of which I had the means of verifying in several places. The heights in this and all other cases are taken from the Ordnance datum, viz. mean tide at Liverpool, which is 12$1⁄2$ feet lower than the Trinity datum of Mr. Mylne's map.

The valley-gravels proper of this neighbourhood, which for our present purpose may be roughly, though not accurately, described as the gravels lying below the 100-feet level, have been divided by Mr. Whitaker into three terraces, which it may be advisable here to describe, viz.:—a high-terrace gravel, occupying the shoulders and sides of the valley at a height of from 50 to 90 and 100 feet above the datum; a mid terrace, from 20 to 30 feet high, in the bottom of the valley; and a low terrace, occupying the low ground in the salient bends of the river, at an average height of from 10 to 20 feet.

Commencing with the north side of the river, he traces the margin of the uppermost river-terrace, or high terrace, as it may be convenient to call it, in contradistinction to the high-level gravel of Mr. Prestwich—from Drum Lane, north of Brentford, passing a little below Gunnersbury to Acton and East Acton, where this terrace ends off; and the next, or mid terrace, runs up at Wormwood Scrubs as far as the London Clay.

The northern boundary of the high-terrace gravel extends to beyond Hanwell on the west, passing by Ealing to Acton, where it is cut by a strip of the London Clay in the ravine of the Acton brook, dividing it from the isolated patch of high-terrace gravel to the eastward, which forms the particular subject of this paper. The average height of the northern limit of the high terrace is about 100 feet.

From Kensington to the Serpentine the high terrace is again found, rising at Campden Hill to the height of 129 feet, and extending beyond the Bayswater Road to the northward. Eastward of the Serpentine it rises again, extending from Piccadilly to Regent's Park, and from Paddington on the west to Finsbury on the east.

All the country to the north of this terrace, with the exception of a patch of gravel in the valley of the Brent, north of Twyford, and some other patches beyond the limits of the tract under consideration, consists of the London Clay; and strips of the London Clay also run round the patches of high-terrace gravel along the sides of the Thames valley, at an average height of 50 feet, dividing it from the