These remarks with respect to defence-walls on the shores composed of resisting rocks apply equally, or rather a fortiori, to the defences required for formations of a softer nature. The action of the sea upon them is, however, of a rather more complicated nature than that which takes place on granitic, or crystalline shores; for it will in many eases be found that at the feet of limestone, or clay cliffs, there are large deposits of shingle composed of tho detritus of harder rocks, carried forward by the force of tho currents, and of the tides, according to peculiar laws. If this shingle should only advance slowly it may tend to protect the surface of the shore, by forming as it were a matlass on which the waves might break without affecting the subjacent rocks ; but if, on the other hand, the shingle should have any tendency to lateral displacement, it will materially assist the abrading action of the littoral currents by tho friction it exercises at the base of the cliffs. The outlines of a limestone, or of a clay, shore often bear traces of this action; and they exhibit cliffs which are nearly vertical iu their upper portions, but hollowed out about the zone of tidal action ; at their feet there is usually a shingly beach, in which the stones have been abandoned by the receding waves according to the specific gravities of the stones themselves, or, in other words, the largest pebbles are always at the top of the beach. The defence of shores of this description may either be effected by walls with aprons, as before-mentioned, or by the construction of stonework, or of timber, groynes, placed at right angles to the advance of the shingle, in order to fix the latter. Groynes are in fact dwarf-walls, projecting more or less above the face of the beach, and continued to about the line of low water ; and they act by retaining the advancing shingle in tho re-entrant angle, until it has accumulated to such an extent as to be carried round the point, or over the top of the groyne. In great storms the groynes arc often laid bare, and there is danger of their being then undermined; and, indeed, the actions to which they are exposed are so violent, that it may fairly be laid down as a rule that it is preferable to form them of cheap, temporary materials, than to form them in a permanent manner.
The consolidation of shores of soft alluvial formations may be effected in various manners, according to their outlines under the action of the currents. If the natural inclination should happen to forum a very small angle with the horizon from the line of the low water, it would generally bo desirable to crown the top of the slope by a vertical wall, of either masonry or of timber. In Holland the dykes are executed of earth work, sometimes with rather an abrupt talus towards the sea, which talus is protected by stone pitching; at others the slope is made with a much flatter angle, and two or three parallel rows of piles, standing up above the face of the slope, are inserted for the purpose of breaking tho force of the waves; and at others, tho face of the earthwork is protected by a species of wall composed of bundle.
of reeds, famines, &c. The hearts of these embankments are formed with reeds and fascines, and especial care is taken to attach them firmly to the subsoil, as well as to secure an efficient system of back drainage. Many instances occur in which the banks of the more protected polders of If ollaud and Flanders are constructed in the same manner as coffer-dams, that is to say, by means of puddle banks rammed in between sheeting enclosures ; and near Havre, in the bay ef the Seine, the fore-shore is defended by a regular timber stockade formed of whole timber piles, whales and back ties, and of horizontal plank ing spiked to the outer faces of the piles; earthwork is carefully rammed at the back of these planks.
It is necessary to guard against the destructive chemical action of the sea-water upon the materials employed for any of these systems of shore defences ; and when the soft limestones, or wood, are employed, to take seriously into account the ravages which may be committed by the various descriptions of boring conchifera3, and worms. The pholas, solen, and lithodomus attack the calcareous atones; the teredo navalis and the lymexylon attack wood, in the temperate latitudes of Europe; in tropical climates there are other species of animals of even a more destructive nature still, which feed upon, or burrow in, both those chases of materials.
It may be as well to add, that in the cases wherein the Dutch engineers content themselves with sand dykes, faced with reeds, the minimum dimensions they employ are about as indicated in the sketch ; but in very many instances they make the sea-slope with an inclination of even I in 20, when the natural profile of the foreshore presents a very flat incline. With the very light materials dealt with in these cases it is indeed preferable to conduct the waves op a long easy slope ou which they can exhaust their power, than to attempt to oppose an abrupt resistance to their shock. When stiff clay is the material used for the banks, and no protection is given, the slope is made equal to 1 in 10; when light sand is used, the slope is made 1 in 20, whatever may be the profile of the foreshore. At the mouth of the Thames and Medway the marshes are defended by slopes of 1 in 4, protected by a concrete bed covered by stone pitching, laid between rows of stakes driven firmly into the banks, and between rows of piles at the top and bottom. (See 31inard, Ouvrages Hydrattliques des Ports de 3Ier, &c.')