SINKING OF THE SOIL. 415 
ments, the old interior dikes are suffered to remain, both as an 
additional security against the waves, and because the removal 
of them would be expensive. They serve, also, as roads or 
causeways, a purpose for which the embankments nearest the 
sea are seldom employed, because the whole structure might 
be endangered from the breaking of the turf by wheels and 
the hoofs of horses. Where successive rows of dikes have been 
thus constructed, it is observed that the ground defended by 
the more ancient embankments is lower than that embraced 
within the newer enclosures, and this depression of level has 
been ascribed to a general subsidence of the coast from geo- 
logical causes ;* but the better opinion seems to be that it is, in 
most cases, due merely to the consolidation and settling of the 
earth from being more effectually dried, from the weight of 
the dikes, from the tread of men and cattle, and from the 
movement of the heavy wagons which carry off the crops.t 
* A similar subsidence of the surface is observed in the diked ground of the 
Lincolnshire fens, where there is no reason to suspect a general depression 
from geological causes. 
{ The shaking of the ground, even when loaded with large buildings, 
by the passage of heavy carriages or artillery, or by the march of a body 
of cavalry or even infantry, shows that such causes may produce important 
mechanical effects on the condition of the soil. The bogs in the Netherlands, 
as in most other countries, contain large numbers of fallen trees, buried to a 
certain depth by earth and vegetable mould. When the bogs are dry 
enough to serve as pastures, it is observed that trunks of these ancient trees 
rise of themselves to the surface. Staring ascribes this singular phenomenon 
to the agitation of the ground by the tread of cattle. ‘* When roadbeds,” 
observes he, ‘‘are constructed of gravel and pebbles of different sizes, and 
these latter are placed at the bottom without being broken and rolled 
hard together, they are soon brought to the top by the effect of travel on 
the road. Lying loosely, they undergo some motion from the passage of 
every wagou-wheel and the tread of every horse that passes over them. This 
motion is an oscillation or partial rolling, and as one side of a pebble is 
raised, a little fine sand or earth is forced under it, and the frequent re- 
petition of this process by cattle or carriages moving in opposite directions 
brings it at last to the surface. We may suppose that a similar effect is 
produced on the stems of trees in the bogs by the tread of animals.”—De 
Bodem van Nederland, i., pp. 75, 76. 
It is observed in the Northern United States, that when soils containing 
