river itself would be principally excavating the Chalk which had been freed from the marine shingle, and would therefore be forming the gravel in its bed, for the most part, from flints derived from the Chalk.
In the same manner, pebbles brought from a distant part of the country, and higher up the river, would eventually become more abundant in the deposits near its mouth, than they were at the first. Still no amount of transport of this kind could bring any pebbles into the bed of the river, which did not, in some form or other, exist within its drainage area.
Besides the transporting power of water, which by itself is, under favourable circumstances, capable of producing considerable excavations in a comparatively short period, there is another force at work, where, as has been supposed in this case, the climate is severe, which not only aids in the transport of pebbles and blocks of stone from one part of the bed of a river to another, but is a fertile source of floods. This is the formation of ground-ice. Sir Joseph Prestwich,[1] in his second "Memoir on the Flint Implement-bearing Beds," has given numerous instances of the transporting power of this agent, and shown the method of its occurrence in running streams, when the cold suffices to reduce the temperature of the water, and of the bed of the river itself, to the freezing point. Under such circumstances a gravelly river bed—and on mud alone, ice rarely forms—may become coated with ice, which being lighter than water will, on acquiring certain dimensions, overcome the forces which keep it at the bottom, and rise to the surface, carrying with it all the loose materials to which it adhered.
M. Engelhardt,[2] director of the forges at Niederbronn, in the Vosges, has, perhaps, more minutely than any one else investigated the causes of the formation of ground-ice; and to prevent its effects in causing floods, actually removed each year from the bed of the stream supplying the motive power to his works, the stones and other extraneous bodies round which it was likely to form. His account of the effects of ground-ice in causing floods in the upper part of the Rhine and the Danube is worth transcribing. These two rivers having "a rapid current, do not freeze, like the Seine, by being covered with a plane and uniform stratum; they bear along large blocks of ice, which cross and impinge upon one another, and becoming thus heaped together, finally barricade the river. It is a grand spectacle, when the Rhine is thus charged,
