are many gasteropodus shells, such as Euomphalus, Pleurotomaria, and many others. According to Professor Edward Forbes, the colour bands, which often ornament the surface of our coast shells, cease beyond a certain depth, and the deep sea shells are colourless. Now, in the neighbourhood of Buxton, Derbyshire, fossil pectens are often met with, still retaining their ancient colour-bands; here we have a fact which proves that, at the time of their entombment, the sea averaged only a certain depth at that place. The Rhynconella and Terebratula were very probably deep sea shells, and did not inhabit the shallow waters. The beak of the Terebratula was perforated, perhaps to allow the extension of a peduncle, whereby the animal was enabled to moor its little dwelling. The perforation in the beak is the chief character of the Terebratulidæ, whence their name. One of the rarer genera of fossils, met with in the carboniferous limestone, is that of the Trilobite. These creatures must have literally swarmed in the Silurian seas, but they became extinct with the limestone, which proved the tomb of the race, for they have not been met with since; a few specific forms only are found in the carboniferous limestone, and these are very degenerate in size. Numerous as are the fossil remains met with in the limestone, still the microscope reveals the astonishing fact, that where it seems to be unfossiliferous, it is composed of the remains of myriads of animalculic forms; in short, however we may examine the structure of this interesting deposit, it gives unmistakable appearance of having passed through the great laboratory of life.

In the neighbourhood of Bristol the limestone is found of various shades of grey and red. In Derbyshire it is often interstratified with numerous layers of black and white chert, and also with beds of greenstone or basalt, which have been erupted, and flowed over their present position, when these rocks were being slowly elaborated on the old sea-bottom. In the Isle of Man the limestone rests upon violently contorted beds of volcanic ash and greenstone, and is considerably harder than either the Derbyshire or the Yorkshire series. The carboniferous limestone in North Wales is somewhat analagous to that of the North of England, alternating with shales and sandstones. This deposit is famous for the prevalence of caverns, fissures, and subterranean rivers, particularly in its thickbedded lower part. It is also exceedingly rich in various ores of metal, especially lead and zinc, which are found chiefly as sulphides and carbonates. As before stated, the carboniferous limestone becomes

H

split up in the northerly direction into beds of sandstones and shales, with occasional seams of coal. These seams vary in thickness from five and six inches to six feet. In the Mid-Lothian coal field one of these is extensively worked, and goes by the name of the "North Green's Seam." This seam varies in thickness from only a few inches to fully five feet. These coal seams, however, are very variable in thickness, not being often continuous over very great areas. The Burdie-house limestone is evidently of fresh-water origin; its shales contain numerous impressions of land plants and fresh water shells, often indiscriminately mixed, but still giving signs of a quiet deposition. The study of the carboniferous limestone in Scotland thus becomes very complicated, and is not so well known as its more southerly type. This deposit is also a very interesting one both to the paleontologist and mineralogist, on account of its numerous organic remains, and the variety of its crystalline forms.

The vast quantities of limestone which occur scattered over different parts of the earth have given rise to the query, whence could all the lime have originated? Various theories were propounded by the earlier geologists to account for it, but the method of true investigation, which Sir Charles Lyell has laid down, has done much to replace these wilder theories by sober facts. Limestone, when pure, contains 96 per cent of carbonate of lime; but passes, by admixture of other substances, into magnesian and bituminous limestone. Sea water contains but 120 of its weight of carbonate of lime; yet this quantity, although appreciable in a pound of water, is the only source from which myriads of marine molluscs and corals are supplied with materials for their habitations.* Now, when we consider the vast quantities of shells which have been manufactured from the above source, and the gigantic coral reefs, ranging for hundreds and even thousands of miles, and extending downwards to thirty and even a hundred fathoms,—we hardly need be at a loss to see how the old limestone rocks have been elaborated. But further, the absorption and secretion of limy matter is not confined to the larger inhabitants of the deep; myriads of animalculæ, invisible to the naked eye, are at work, and their deceased remains sometimes form beds of enormous thickness. So fruitful are these creatures in their procreation, that some of the ports in the Baltic have been literally blocked up by

*Fowne's "Elements of Chemistry."