620' THEORY AND PRACTICE OF ENGINEERING. Bqox II.
and slip. This water should, therefore, be collected as much as possible at the surface, andcarried away by drains.
Clay in a moist situation requires a slope of at least 3 to 1, to enable it to stand ; whenkept drv, 2 to 1 has been found sufficient. Where sleepers have been laid upon it in minesor at the bottom of tunnels, they frequently rise up, and are broken, by the mere expansionof the clay; this occurred in the tunnel of the Manchester and Bolton railway. Housesor buildings of any kind placed upon it are in constant motion : when the weather is dry,the foundations contract, when wet expand, so that the walls are never at rest; when theclay is moist, a heavy wall will sink, and continue to do so until out of the influences thatproduce these changes.
Whenever a slope is made at the side of a cutting in clay it is advisable to have at thetoe a footing of concrete or a wall built up to guard against the mass sliding from it. Thedifference in the colour of the yellow clay and the blue beneath arises from the iron theycontain. When this is excluded from the air, it is found as a protoxide ; in the upper clayit is a peroxide.
The London clay is more or less pervious abounding with fissures in all directions,many of which hold a slimy earth and abundance of water : this often passes down to thelower stratum on which it rests, which, if composed of sand, is washed away, and then thewhole superincumbent mass either settles or moves forward ; it is supposed to contain 10per cent, of water. Where canals have been cut through clay, and the embankmentsformed of the material thrown out, they have remained firm as long as their weightbalanced the upward tendency of the water in the substratum of the bed of the canal: butwhen the increased weight of the mass destroyed that equilibrium, the embankments sunk,and forced the bottom upwards. 'Hie greatest depth of the London clay is estimated at700 feet, and its greatest elevation above the sea at 760 feet.
Plastic clav belongs to this formation, and is of various colours; it is so called fromits being used in the potteries; it is found in different degrees of thickness, and containslignites, amber, and shells, both freshwater and marine. The number of tertiary fossils isconsiderable ; the greater proportion are terrestrial ; the fishes are so nearly related toexisting forms that it is difficult to class them, crocodiles and snakes agreeing with thosemet with at the present day.
Upper Chalk had its surface furrowed by the action of the waves and currents before anyof the sands or clays were deposited upon it, deep indentations being frequently observed, inwhich are imbedded sand, gravel, and flint.
The stratification of chalk is often obscure, where layers of flint are wanting; where theyoccur they form strata of from 4 to 7 or 8 inches in thickness, and the distance betweenthese beds varies from 2 to many feet.
Lower Chalk is harder and less white than the upper, and is sometimes varied by greengrains, generally with fewer flints, and often without any. The organic remains of thecretaceous system are nearly all marine; the plants are few, and exclusively marine, withthe exception of fragments of coniferous trees, which have evidently floated in the sea, assome specimens have been perforated by Teredines.
Among the fossil shells are the Terebratula*, which live in deep and tranquil seas, andthe extin't species of Crania and Catillus, the Belemnite, Ammonite, IJaculite, and Tur-rilite, of the family Cephalopoda , which resemble the cuttle-fish. Sea Urchins, CoralsSponges, &c., are also dispersed through the chalk and flint; the latter of which owetheir irregular shape to the inclosed zoophytes, as the hollows on their outer face arecaused by brandies of a sponge, which may be sometimes discovered by breaking the flint.
Fish and Crustacea are found, but no bones of land animals, terrestrial or fluvial shells,plants, sand, or pebbles, indicating a formation in a deep sea at a great distance from land.Fossil fishes, with the air-bbdder distended, have been discovered in the Sussex chalk,leading to the conclusion that their destruction and envelopement were sudden.
The chalk formation in England is of great extent; it is chiefly of carbonate of lime in afine granular state; the greatest height which it attains above the level of the sea is about1000 feet.
Greensand generally forms the base of the chalk hills, and consists of small grains ofsilicate of iron agreeing in composition with chlorite; the greensand deposits are a suc-cession of ordinary beds of sand, clay, marl, and impure sandstone, probably the detritus oiformer rocks, the nature of which may be traced in the pebbles of quartz, quartzose sand-stone, jasper, flinty slate, and grains of chlorite and mica.
The upper greensand is a formation of sand and loam, frequently mixed with chert,which passes downward into clay and marl called Gault Below the gault is the lowergreensand, which is partly ferruginous sand and sandstone, with some limestone; amongthe latter may be mentioned the Kentish rag, used for building and repairing the roads, andamong the sandstones that called firestone, which derives its green hue from grains of sili-cate of iron.