Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/228

110 110 PROCEEDINGS OF THE GEOLOGICAL SOCIETY.

The first gradient is 1 in 33, north; 1 in 90, 1 in 100, 1 in 105. 1 in 110, 1 in 110, 1 in 110, 1 in 57, 1 in 60, 1 in 70, 1 in 60. Here it crosses the janction of two roads at a height of 155 feet abore the sea. Then follow on north 1 in 60, 1 in 27, 1 in 40, 1 in 60, to the point 0, at a height of 120 feet above the sea ; then 1 in 30 and 1 in 75 to the railway, 1 in 33 to the Imperial Eoad, then 1 in 56, 1 in 50, 1 in 231, and it crosses the top of the escarpment of loess at a height of 81 feet above the sea. Then the face of escarpment falls 16^ feet in 18 feet, then is horizontal to the river.

If a line be drawn from the Point N to the Eiver Somme along the line ]Sr P, it will pass under the junction of two roads at a height of 142 feet above the sea, or 15 feet under the roads. It will pass 10 feet above the rails, and 2 feet above the Imperial Road ; so that the extreme convexity of the surface at any point of the line of 7458 feet is only 15 feet. This is important, as the section has been re- presented as enormously convex by previous writers.

The surface of chalk at the junction of the two roads is 142 feet above the sea, and is therefore only 6 feet above a straight line drawn through NOP. The surface at the junction of the two roads is very slightly convex. The surface of the chalk at the railway is 23 feet. below a straight Kne drawn from the Perme de Grace 201 feet above the sea to the Somme (at a height of 61 feet above the sea) ; a straight line from the Ferme de Grace, 201 feet above the sea to the Somme, 61 feet above the sea, passes over the railway 8 feet above the rails, and 23 feet above the surface of the chalk at that point ; so that the surface of the chalk is concave to the extent of 23 feet. At the Imperial Eoad the surface of the chalk is concave to the ex- tent of 22 feet, although on the upper part it is convex to the extent of 15 feet.

lY. Characters oe the Chalk, Gravel, axd Loess.

I will not trouble the Society with the details of Section N Q (Plate lY. fig. 8), but wHL now proceed to describe the characters of the chalk, the gravel, and the loess, as I have observed them near Amiens.

1. The, CliaTk. — The condition of the chalk itself near Amiens is remarkable in some places.

In a railway section near Pont de Metz, about three miles from Amiens andMontiers, the chalk surface slopes northward at an angle of 20°, and is overlain by 20 feet of dnft sands dipping 10° N. where they touch the chalk, but filling up the concavities of the chalk, and.having their upper surface sloping northward at an angle of 3°.

At Pont de Metz the chalk is covered with a drift chalk-marl, and with beds of chalk rubble and chalk pellets, with very Kttle mixture of sand or clay, 15 to 20 feet thick.

j^ear Guigencourt, a quarry in the chalk on the plateau, about four miles south of Montiers, the chalk is very much split up by joints lying at an angle of eighty degrees, or very nearly vertical, and also nearly at right angles to the planes of bedding of the chalk. (Fig. 2.)