Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/230

112 112 PEOCEEDIXGS OF THE GEOLOGICAL SOCIETY.

The drawings of the chalk-quarry, fig. 2, and in M. Dailli's gar- den, north of C, fig. 4, will explain the remarkable character of the decomposition that has affected the chalk. Xot only has chalk been removed by the chemical action which bores pipes in it, but the loess appears to have followed closely, penetrating through the mass for many feet, occupying the vacant space made by the destruction of the calcareous matter in many places, or uniting with it, and making a kind of Combe Eock.

The harder pieces of chalk are left, often in a boulder-Kke form (as drawn), with slightly rounded or abraded comers, the chalk between large pieces then being loose and friable, and marly in colour, often mixed with loess, and with ferruginous stains. AMien the chalk is quarried, the large masses fall down like boulders, and are used for purposes of masonry, untouched by the quarryman. The hard pieces of chalk project beyond the soft matrix in which they are enclosed, like the flints upon the Brighton cliff, making a serrated face. The largest piece that has fallen out is only about three feet long, according to M. Dailli, who has quarried thousands of tons of chalk without meeting with a larger mass. There is a pipe, ten feet in diameter, in M. Dailli's garden, and the depression in the chalk at the north-east comer has a pipe-hke form.

The lines of large flints which traverse the whole of the escarp- ment horizontally are perfectly in situ.

That this decomposition of the chalk t?t situ has some relation to the physical circumstances following the deposition of gravel at St. Acheul is, I think, probable, as some part of the drainage from above St. Acheul would pass through the escarpment in question in order to get to the marshes, and the action which has caused the removal of the chalk must have acted with great intensity on the high land ad- joining, so that the current was fi^om above downwards. About one- eighth of the St. -Acheul gravel consists of chalk in the form of large pieces averaging 4 inches diameter, ofc halk pellets from J to 1| in diameter, and of chalk finely divided and mixed with clay.

"Miere we can see the chalk near C, it is so perforated by pipes and separated into small pieces that it seems prepared for a rapid denudation if attacked by water with any vigour ; and if this was the condition of the chalk also at higher levels near St. Acheul and Montiers, we can account for the large quantity of chalk contained in the Amiens gravels.

The fall of the Somme from Longueau to Montiers is fifteen feet, the river flowing from south-east to north-west nearly, at a gradient of only 1 in 1520. The rails are 96 feet at La Xeuville, and 99 feet at Montiers, above the sea-level.

By referring to the sections, C D, E P, G H, which are parallel with the Eiver Somme and the Imperial Eoad, it will be seen that on a line from east to west, 1644 yards long, from the eastern escarpment of the chalk east of St. -Acheul to the western escarp- ment of the chalk near the northern termination of the Eue de Cagny, the surface of the chalk is extremely regidar and horizontal.