Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/239

Rh TYLOR AMIENS GTIAVEL. 121

section, page 257, Phil. Trans. 18G4, the height of the rails at Montiers is marked 130 feet ; it should be 99 feet, according to M. Guillom. It is to be regretted that Mr. Prestwich was supplied with incor- rect figures of the relative levels of the ground about Amiens, as the introduction of such errors in the section must have materially affected Mr. Prestwich's theoretical views, as he says, " The upper section at Montiers, which I discovered in 1861, was conclusive as to the relative ages of the gravel" (p. 248, Phil. Trans. 1864).

In the plan, Plate v., Phil. Trans. 1864, accompanying Mr. Prest- wich's memoir, the bare chalk is shown as invariably separating the upper and lower gravels all the way from Amiens to Abbeville ; but I have never seen a case of the kind.

It must be remembered that so much gravel has been removed during the last four years, that the sections are now much clearer ; and, with the assistance of the accurate measurements of M. Guillom, present examiners have a great advantagokover their predecessors, in examining the structure of the gravel near Amiens.

I cannot suppose that Mr. Prestwich would now separate the Montiers gravels, seen in and above the railway- cutting at Montiers, from those in the Great Montiers pit, and into two horizons, as there is only a difference of twenty-two feet between the height of the gravel on the top of the railway-cutting and that in the Imperial road. As nearly the whole space between these two points has now been excavated, the continuity of the gravel is now proved.

When Mr. Prestwich supposed that there was a continuous bare band of chalk separating the gravel in the railway-cutting at Mon- tiers from the gravel near the Imperial road, and that the top of the railway- cutting was (according to the measurement in his section, page 257, Phil. Trans. 1864) sixty feet above the Imperial Eoad, he very naturally took a diflPerent view of the relations of the gravels from what we must take at the present time, with the additional information we possess.

The section on Plate lY., therefore, appears to destroy Mr. Prest- wich's argument, on which he has constructed a division of the gravels at St. Acheul and at Montiers into upper vaUey-gravels and lower valley-gravels, of different ages, and situated on different hori- zons, separated, as he supposed, by a band of bare chalk from each other, — the upper valley-gravel being supposed to have been deposited before the excavation of the last fifty feet of the Somme valley, which excavation, he considered, preceded the deposition of the gravels near the Imperial road, Montiers.

The character of the surface of the chalk at Montiers has been discussed at full length in this paper, and shown to be concave at the pits ; while it is represented as highly convex at Montiers by Mr. Prestwich and Sir C. Lyell.

In the long section CD (Plate IV. fig. 3), the St. -Acheul gravel, at a height of 140 feet above the sea, is shown to be separated from the loess at Longueau, at a height of ninety feet, by an escarpment of bare chalk. The tramway (Plate IV. fig. 1), passing from the Im- perial road to the railway, crosses one of the supposed bands of