Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/235

Rh TYLOR — AMIENS GllAVEL. 117

The River Cray falls into the Thames ranch as the River Arve falls into the Somme. The Crayford g-ravel is 100 feet thick, and confined to a space between two valleys, the eastern valley occu- pied by the River Cray, and the other and western a dry valley, like that south-west of St. Acheul.

Bounded by valleys east and west, the Crayford and St. -Acheul gravels lie against escarpments of the chalk parallel to the Rivers Thames and Somme.

I have not presented more than a few varieties of the gravel-sec- tions to be observed in Amiens, for want of room (Plate IV. figs. 12 & 13, and fig. 8 below), and I propose to make some remarks upon the peculiarities of deposition to be observed there at some future time. I will now only observe that the character of the sections, I think, clearly shows us that a large quantity of the gravel material now exposed in the quarries opened for ballast neaf Amiens had its source in the hiUs or plateau immediately adjoining and above St. Acheul and Montiers, and was washed into the valley of the Somme in a direction from south to north, and mingled vtath the materials brought down by the Somme, flowing from east to west.

The quantity of chalk-detritus is about one-eighth of the whole mass of gravel and loess, and makes the Amiens deposit second in importance to the Brighton gravel, as far as the presence of chalk is concerned. The unrolled condition in which the large pieces of chalk in the gravel generally occur proves the local origin of the chalk, and that it has been brought down from the high lands and not thrown up by the river.

We might expect an important diff'erence in mineral character between the gravel and loess at the respective heights of 150 and 75 feet above the sea. I have compared the gravel of St. Acheul, 140 feet above the sea, with that at Montiers, from 70 to 80 feet above the sea, as carefully as I could, in order to find some marked distinctions, but up to the present time without success. I have sketched a piece of gravel at St. Acheul, 140 feet above the sea (Plate IV. fig. 12), and a piece in La Neuville, 105 feet above the sea, and immediately north (fig. 8). There is a great deal of varia-

Section in La NeuviUe Ballast-pit. Loess and Gravel.

tion between these two sections ; but there is still more variety in the gravel-section of a part of St. Acheul, 200 yards to the east, at a height of 145 feet above the sea (Plate IV. fig. 13). Similar species of shells have been discovered and named by Mr. Prestwich and others at Montiers and St. Acheul, at very different levels ; but there

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