Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/242

124 124 PROCEEDINGS OP THE GEOLOGICAL SOCIETT.

bankment to the west of that point for some distance, it is difficult to place Mr. Prestwich's section at any other point than where I sup- posed it was taken, on account of the configuration of the ground. "WTiether there was chalk, or not, at any one point, is quite imma- terial to my argument. I do not find the Montiers section at all as represented by Mr. Prestwich and Sir C. Lyell. (See fig. 12, p. 123.)

The Montiers section appears to be the one adopted as a type of the Somme district, first by Mr. Prestwich and afterwards by Sir C. Lyell. Both authors represent, in several sections of the Somme, a great extent of chalk, separating highly inclined beds of gravel, which they have distinguished in age by its position above or below this outcrop of chalk, as upper- and lower-, or high- and low-level gravels. The sections which I place before the Society appear to me, on the contrary, to show that this distinction is not a real one, but that the deposit of gravel is one and continuous, de- posited in concavities of an ancient chalk valley, and is not highly inclined as represented in the ' Antiquity of Man ' and the ' Philoso- phical Transactions.'

In page 264, Phil. Trans. 1864, Mr. Prestwich gives a theoreti- cal account of the view he takes of the deposition of the gravels. Part of the upper-level gravels are represented as remaining un- touched, while the vaUey is cut down 50 feet, and a newer set of gravels deposited at lower levels ; my sections show that there is no evidence of any such action.

The same views are extended by Mr. Prestwich to the loess deposits ; the loess of St. Acheul is considered a much older depo- sit than the loess at Montiers.

Mr. Prestwich lays great stress, in his paper in the Society's Journal, p. 500, on the valley being too large to admit of the possibility of its being filled with water up to a height of 100 feet above the present water-level.

I have already submitted the argument that we ought to judge of the height of a flood by means of the debris it has left, and not by any theoretical notions of our own.

In 1866, twenty inches of rain feU in Scinde in twenty-four hours, in a flat country intersected by rivers. Mne girders, weighing nearly eighty tons each, were washed off the piers by the Mulleer Eiver from the Pailway JBridge, situated sixteen miles above Kurrachee (fig. 13). This bridge consisted of eighteen girders (see Plate IV. fig. 10.) They were not box girders, but made of wrought ii^on on Warren's system. The bottoms of these girders were sixty-five feet above high-water mark, spring tides, Kurrachee Harbour, and se- venty-four feet above low- water spring tides. They fell in the course of six hours ; and one girder of the weight of eighty tons was carried two miles down the river, and nearly buried in sand*. The section of the river bridge is represented (Plate IV. fig. 10). The fall of the Mulleer Eiver only averaged ten feet per mile for fifteen miles

sent at the Meeting, and confirmed this statement, which he had previously given me.
 * Mr. John Brunton, F.G.S., Chief Engineer of the Scinde Eailwaj, was pre-