Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/22

Rh previous sea bottom, and inhabited by marine animals suited to the new arenaceous condition; and we might also expect to find the remains of estuary creatures occasionally in the sand. Above this second or sandy accumulation, the detrital matter getting above the surface of the sea, the results above mentioned, as arising from the united action of the breakers and the river, would be obtained. We should not expect a thick deposit of this kind, but during its progress there would probably be a mixture of estuary remains, even perhaps lacustrine, (isolated fresh-water lakes being formed amid the lagoons silting up,) with the marine shells, accumulated in the sands by the breakers. In shorter rivers, flowing into deep water, of which there are numerous examples on the coasts of the Mediterranean, and especially when such rivers are in flood, light detritus is borne outwards to settle in the deeper parts of the sea, fine sand even being so carried out in mechanical suspension; while the chief accumulations are formed of sand and pebbles, forced forward over the bottom of the river, and deposited, layer after layer, in a general diagonal form, the surface keeping, as a whole, horizontal. From changes in the direction of the river's mouth, or mouths, these accumulations take a complex form; so that a section made horizontally, a few feet from the surface, would possess somewhat of the following character,—one due to the varied protrusion of sands and pebbles, according to the direction and force of the water acting on the loose materials; so that a vertical section of the accumulation would be much as represented in Figs. 4 and 5.

In these cases, also, during the advance of an embouchure, the sands and gravels could cover bottoms in which mud and clay had previously been formed by deposit from mechanical suspension being, as it were, swept over them.

We have only to suppose alterations in the relative levels of the sea and land, such as are now well known to have taken place, to have alternations of days, sands, and gravels entombing the remains of