Cnxr. I.

GEOLOGY.

625

Septaria. In some localities these occur at upwards of 200 feet in depth. The shells ofthe Paris basin are also found in it, but there are no remains of terrestrial mammalia,although there are those of the tortoise and crocodile, which indicate the deposition to havebeen made at no great distance from land ; many of the shells found in the clay bear astrong resemblance to the testaceous fauna of the tropics, though they cannot be identifiedwith any species now living. The fish found in it appear to have belonged to a warmclimate: among them is the sword-fish, Tetrapterus prise us, about 8 feet in length ; and thesaw-fish, Prestis bisulcatus, 10 feet in length, both of which were found in the Isle of Sheppey,where more than fifty other species of fish have been discovered; the nodules of calcareousstone in that island are formed into Roman cement, and the fossil Flora resemble those onthe shores of the Mediterranean. The shells are the Dentalium striatum, Paludinuinlenta, Crassatella sulcata, Venericardia planicosta, Conus scrabriculus, Voluta dubia, &c.

The London and Croydon Railway, during its execution, laid open a complete section ofthe London clay, down to its junction with the plastic clay. At New Cross the plasticclay was decomposed sandstone, with fossils, black sand, clay and sand, occupying a depthof a little more than 80 inches; then a layer of strong blue clay of 10 inches or more;then a layer of loose ferruginous sand, about 1 foot in depth ; afterwards fine sand to thedepth of 2 feet or more; then a stratum of flint shingle, of the same depth ; on this liesthe London clay, in many places more than 15 feet in depth, and above it the top yellowclay.

'Hie cutting near to that at New Cross is nearly 80 feet, and the rails are laid on thetop of the plastic clay. About three or four years ago a great movement was observedin the embankment of the western slope, which forms the inside of the curve, taken bvthe line of rail: the yellow clay became, from the great fall of rain, in a semi-fluidstate, and was in motion throughout its whole thickness; in a few hours more than50,000 cubic yards slipped on the surface of the blue clay, and covered the rails for aconsiderable distance. The blue clay is stiff and insoluble, and so compact that it isimpervious to water; but the yellow clay above is mixed with a variety of other mattersoluble in water, as lime, fullers’ earth, and bands of septaria; these being dissolvedoccasioned faults and fissures within, which became filled with water, caused an ex-pansion of the mass, and at last its falling down, from not being able to sustain its ownweight on the steep slopes at which it had been cut. The strata also dipped in a mannerto favour the slipping, which chemical action may have aided, for the iron pyrites foundin the yellow clay may have been decomposed by exposure to the atmosphere, and sul-phuric acid evolved, which, entering into combination with the carbonate of lime, wouldform crystals of silenite, materially affecting the bulk of the whole mass, and assisting theseparation of the clay.

To remove this slip stages were erected at each end, sufficiently high to permit thewaggons to run under them, and advance gradually into the slip, a way being cut byworking a gullet down to the rails. Two sets of waggons were made use of, which, whenfilled, were drawn away by locomotives to the nearest embankment, where the contentswere either carted or wheeled to their destination. When the whole was cleared, a cuttingwas commenced at each side where the slips took place, and after removing above 250,000cubic yards of clay, the slopes were trimmed back to what was considered a safeinclination. This was effected by cutting benches and intermediate slopes ; on the westside there are three benches, and on the other two; these benches vary in dimensions upto 65 feet in breadth, and the several slopes are cut with an inclination of 2 to 1.Drains are formed both on the benches and in the slopes, to carry off the surface water.

Some of the earth on this line is very untractable: at Forest Hill a continued rainresolved it into mud; and after all attempts to drain it were found ineffectual, a bench70 feet in width was cleared about 20 feet up the slope, and on this 100,000 cubic yardsof clay were run ; at the back of the benching was a retaining wall of gravel, nearlydouble its width, and varying from 5 to 12 feet in height; the clay taken out was thenthrown in front of it, to gain additional weight, and a greater firmness in the soil has beenthe result. Drains of gravel seem to be in most cases efficacious, the water, which is thegeneral cause of all slips, being by this means entirely drained off; care is also required tosecure the foot, and to make the slopes in such soils from 3 and 4 to 1.

Retaining walls on the London and Birmingham line, near the Euston Square station,were forced inwards from the expansion of the clay behind them: they were of brick, 5 feet6 inches thick at bottom, and 2 feet 6 inches at top, with a curved face, and whenever takendown for the purpose of repairs, the face of the clay appeared to stand perfectly straight,the expansion and the force obtained by it being attributable to the action of the atmo-sphere, which causes a constant contraction and expansion, if the clay be exposed even for afew hours ; it is from the same influence that the sides of wells sunk in it so frequentlygive way ; when dried, it occupies a seventh or eighth less space than when in its bed.During the heat of summer it will shrink, and form cracks or fissures, which, when the rainfails, become filled with water, the hydrostatic action of which occasion masses to break off

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