624

THEORY AND PRACTICE OF ENGINEERING.

Book If.

phurct of iron and clay ; the whole being subjected to long abrasion, and exposure to theoxygenating processes of the atmosphere.

The Argillaceous Sediments consist of a variety of light greenish and blue marls, with bedsof gypsum, and those which are found accompanying the coloured sands of Alum Bay, inthe Isle of Wight, are singular from the variety as well as richness of their tints.

The limestone is soft, marly, and generally abounds in shells. The whole of this forma-tion was in all probability derived from the detritus of those rocks that surrounded theseas where they were deposited ; or different rivers emptying themselves into the sameestuary might each bring with it sedimentary matter taken up in its course, either argil-laceous calcareous, or siliceous, more or less finely comminuted. 'Hie velocity of theseveral streams would influence the deposits, and distribute them at several distancesand depths; but in all cases the tertiary formation is the result of stratiform deposits,and the most striking features consist in the repeated alternations of marine and freshwaterbeds.

The marine formations of this series are divided into Newer Pliocene , Older Pliocene ,Miocene, and Eocene , and alternating with these there intervenes a fourfold series of otherstrata, or freshwater formation.

The Newer Pliocene consists of lacustrine and fluviatile deposits, and the majority of shellsfound in it bears a great resemblance to those of the living species ; this system in Sicily alternates with volcanic beds, and is constituted of hills, rising in some cases upwards of2000 feet above the level of the sea; and the species of shells found in them arc similar tothose inhabiting the Mediterranean Sea .

The great limestone formation, which is the uppermost of these strata in Sicily , bears someresemblance to the calcaire grossier of the Paris basin, and consists of a yellowish whitecalcareous rock, often containing leaves of plants, reeds, Sec. as if a river of fresh waterwith carbonate of lime in solution had floated down these vegetable remains. Its thick-ness is sometimes 800 feet; in some situations it is of a more compact character, and mayhave been precipitated from the waters of mineral springs.

The Older Pliocene. — The Norfolk Crag formation comes under this division, and re-sembles a raised sea beach; it is composed of layers of sand and pebbles, mixed with marineshells, and occupies much of the low ground of the eastern coast of England; the pebblesfound in it are much worn and abraded, and resemble those on the margin of a sea in-fluenced by tides. This crag is usually considered the uppermost of the British strata, andconsists of several layers.

The Miocene comprises the Red and Coralline Crag, the latter being formed of calcareoussand, derived from corals in a state of decomposition, among which are embedded sponges,corals, and shells in good preservation: this formation in Suffolk is quarried and used forthe purposes of building; it is not stratified, its bed does not exceed 10 or 12 feet inthickness, and it is of a soft nature. The Red Crag, which is uppermost, has a deepferruginous colour, and is formed of layers of siliceous sand, mixed with broken shells,and was probably* cotemporaneous in its formation with the coralline : among the shellsare the genera Buccinum and Murex, exclusive of Polypi, Radiaria, and Crustacea ; somehundreds of species of invertebrated animals are found, among which are Annulata, Cirrhi-peda, Conchifera, and Mollusca . In the Suffolk Crag is the Fusus contrarius and theFusus bulbi; and in many parts of the cliffs of Norfolk and Suffolk , sand and shinglealternate without any organic remains ; whilst in others there are abundant remains of thebones of terrestrial quadrupeds, as well as Ammonites, vertebra of Ichthyosauri, and driftwood, sometimes lying in confused masses, at other times in regular strata. From themixture of these terrestrial and marine remains, there can be but little doubt that the cragformation was partly deposited at the mouth of a l iver: the bones of the mammalian re-mains are ascertained to belong to the leopard, the bear, the hog, and a large kind of deer;some of the Mammalia of the Norwich crag are of a more recent date, such as the teeth ofthe Mastodon longirostris, the elephant’s tusk, the bones of the horse, the hog, and the fieldmouse, mixed with those of birds and fishes.

Eocene . — In this formation may be classed the Bagshot sands, which are of an ochreouscolour, and are found alternating with foliated green clay, green sand, and various colouredfoliated marls ; a few shells are occasionally met with in the marls, of the genera Trochus,Pecten, and Crassatella ; though organic remains are rare, remains of fishes do occur ; thegreatest elevation above the level of the sea is 465 feet.

The fresh-water formations of the Isle of Wight are divided into an upper and lowerdeposit, the bed separating them containing marine remains; the shells belonging to generaMurex, Buccinum, Natica, Venus, Nucula, and Corbula.

The upper fresh-water formation, composed of yellowish marl, contains shells of thegenera Limnea, Planorbis, and Helix.

London Clay has either a bluish or black colour, and sometimes passes into calcareousmarl, having a depth of more than 1000 feet in thickness. Argillaceous limestone con-taining calcareous spar occurs in numerous layers, as well as ovate or tlattish masses, called