Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/216

106 character, with those of other rivers, of which I have had surveys made; and I hoped to have brought the whole subject of valley-gravels before the Society in the early part of 1868. The announcement of the intended absence from England of Mr. Prestwich has induced me to describe the Amiens gravels first, in order to have the advantage of his presence at the meeting, reserving my account of other river-gravels and general comparison to a future paper.

The plan and sections of the Quaternary deposits at Amiens illustrating this paper will, I hope, make the actual geographical and geological positions of the Amiens beds quite clear.

It should be mentioned that, in August 1866, after examining the levels myself, I called at the railway office at Amiens to obtain the precise height of the different points on the railway above the sea, and to get the name of a competent surveyor, and was referred to the chief engineer, M. Guillom. That gentleman only saw me for a few minutes, but kindly promised that if all the points on which I required information were laid down on a plan, he would have the sections carefully taken for me in a few weeks. This plan was accordingly sent to him by me a few days afterwards, with the lines marked on which I wished the levels.

The work was more tedious than was expected by M. Guillom, and he did not send me the measured sections until May 1867. The levels of these sections have been taken with the greatest care, and, I believe, are as precise as any that have been taken for geological purposes; and I am therefore indebted to M. Guillom for the means of drawing an exact picture of the surface of the chalk prior, as I believe, to the deposition of any of the valley-gravel or loess at Amiens. The value of this communication therefore much depends on M. Guillom's survey.

The heights on the maps and sections are in English feet, the datum line being mean tide at Havre. The scale of the plan (Plate III.) is 3$1⁄6$ inches to a mile. The vertical scale in Plate IV. is $1⁄2$ inch to 55 feet, and is three times the horizontal scale. The dotted lines on the plan show the position of five sections, I K, R S, L M, N O P, N Q, nearly at right angles to the river, some of them extending to a height of 200 feet above the sea. The line A B is along the Imperial Road. See Plates III. and IV.

There is also a longitudinal section, divided into three parts on account of the public buildings at St. Acheul preventing continuous levels being taken. The divisions are C D, E F, and G H; but it will be treated sometimes in this paper as one section, C H. See Plate IV. figs. 3, 4, and 5.

It passes through the celebrated pits of St. Acheul. and is bounded by the River Arve, a tributary of the Somme, at its eastern point, C, and by the escarpment of chalk in the Rue de Cagny (700 yards west of the railway-station, Amiens) at its western extremity, point H.