Oblique lamination of sandstone beds in millstone grit.

difficult to find the true dip of the rocks. In the neighbourhood of Todmorden, Yorkshire, the millstone grit is interstratified with a band of black limestone, containing species of fossils peculiar to it, and not found elsewhere; along with black shales, very finely laminated, and covered with impressions of aviculo-pectens, goniatites, nautili, and numerous heads and stems of encrinites. The millstone grit takes its name from that variety of stone being generally obtained from it. In South Wales the millstone grit is represented by red, cherty, or pebbly grit stones, which go by the name of "Farewell Rock."

The most important beds of the carboniferous formation, however, are those which go by the name of the upper coal measures, chiefly because the coal-bearing strata are almost limited to them; together with thick layers and nodules of ironstone. These strata give the name to the whole group. They consist, in general, of alternating strata of clay, shales, sandstones, limestones, and coal. The seams of coal are found from mere films to upwards of a hundred feet in thickness. North of the Humber, in the neighbourhood of Scarborough, there occurs, in the lower beds of the oolitic formation, a seam of coal about two feet in thickness, which is there worked; its quality, however, is much inferior to that of the carboniferous formation. Coal is also extensively worked in the oolite at Virginia, North America; but it would appear that these seams are only local, and are not by far so general as those in the coal measures, neither are they, as in the latter case, peculiar to the strata in which they occur.

To account for the vast beds of fuel which are stored up in the rocks of this date, it was supposed by Brogniart and the earlier geologists, that the atmosphere possessed a greater proportion of carbonic-acid than it does at present. There does not seem, however, any necessity for this theory, although it is still strongly contended for by some celebrated geologists; for, "though the proportion of carbon in our atmosphere is only about one part in a thousand, yet it contains upwards of 3,081 billions of pounds of it;

a quantity" according to Liebig, "which amounts to more than the weight of all the plants, and of all the strata of mineral and brown coal existing on the earth." That coal is of vegetable origin, there is now no doubt whatever. All the stages between perfect wood and the most compact bitumen may be traced, and yet we never lose sight of its vegetable origin, for it can and has been traced, even in anthracite, by the American microscopist, Professor Bailey. We have first the anthracite of the lower carboniferous strata, which is generally supposed to be a kind of metamorphosed coal; then the coals of the true carboniferous; next the highly bituminous coals of the oolite; afterwards the lignite and brown coals of the tertiary; and lastly, the blackened and semi-bitumenised logs of our peat-bogs and deltas of the present epoch. The resemblance can also be traced in its chemical composition. Wood consists of carbon, hydrogen, and oxygen. Coal also consists of the same elements, but in different proportions. The following is a table of different carbonaceous substances, which will establish this relation at once.

If we take from wood thirty per cent of its oxygen and nitrogen, under compression it will form coal. The oxygen is often taken up by a portion of carbon, forming carbonic acid, which exists in such abundance in the coal-beds. The drainage of these gases is supposed to harden the coal very considerably. The ancient vegetation, which has formed our coal beds, was buried beneath waters, submerged on the place of its growth, or else has been conveyed to the estuaries, or mouths of rivers, and there sunk in spots alternately occupied by salt and fresh water; where, under the influence of heat, generated by chemical action, and being subjected to a vast superincumbent pressure of sand and mud, the vegetable masses have been converted into coal. "The agency by which this change has been effected is supposed to be similar to the chemical process by which vegetable matter is known to ferment, and sometimes to

produce spontaneous combustion.

If hay, for instance, be stacked

in a moist state, or if it be too closely packed, fermentation and often ignition, is the results. If the process be interrupted, and combustion prevented, the hay is found to have acquired a dark brown colour, and a strong bituminous odour. The same phenomena are observed in the case of flax packed and pressed, whilst in a damp state, which is liable to the same results. All vegetable substances, under like conditions, become exposed to similar chemical changes. Were any vegetable matter in a moist condition placed beneath great pressure, so as to prevent the escape of its gaseous elements, bitumen, lignite, and coal would be produced in the various stages of the process." Naptha, petroleum, bitumen, lignite, jet, coal, amber, and even the diamond are known to be of vegetable origin; the diamond only differs from anthracite in being purer carbon, one being crystallized, and the other not: the diamond may be burnt just like any other vegetable production.

The vegetation which has produced coal, grew in broad and shallow lagoons, or sheets of water, which received at intervals deposits of sediment, silt, and mud, the waste of neighbouring land. These pools, or lagoons, seemed to have been speedily filled up by the rank growth of a profusion of plants, until by a mixture of various ingredients, vegetable remains, sand, and mud, the lagoon became converted into a morass; a fresh vegetable growth of a somewhat different character, now ensued, as may be traced in a single bed of carbonaceous shale; this vegetation consisted of calamites, and other ancient flora, which seem to have delighted in moist situations, and the spoils of these plants have furnished materials for coal and anthracite. This lagoon, by repeated subsidences, during a long course of ages, may have sunk beneath the level of the sea, and thus rendered it a basin for the receipt of alternating deposits of sand, clay, and limy matter, and these have produced the strata of sandstones, limestones, and shales which occur between seams of coal. The alternation of beds of coal with marine deposits, may be explained by supposing that an extensive subsidence of the estuaries, or lagoons, in which they have been formed, reduced them beneath the level of the sea, when the submerged forests were covered by marine sediments, either of drifts of clay, sand, or decayed remains of marine shells. This subsidence, however, must have been very gradual, for the lamina of the beds are of extreme thinness. Some of these shale-beds are exceedingly

rich in the excrements of fishes, coprolites; shewing that the ancient animals lived and died, and followed the usual routine of animal life; where these coprolites, however, most abound, the remains of fishes are most scarce, but when found, are generally in very perfect condition; this shews to us that the waters were fitted for animal life, even when the mighty thicknesses of rock, in which they are now entombed, were slowly forming by the gradual settling of discoloured waters.

A considerable difference of opinion has prevailed as to the circumstances under which coal has become imbedded. This difference of opinion rose to such a pitch, as, at one time, to form strong party feelings, as in the case of the Neptunists and the Plutonists. One party maintained that the original forests of the carboniferous era were swept away by inundations, or carried down by great rivers to be entombed in their estuaries. These drifted trees, becoming water-logged, sank to the bottom, and after passing through the chemical changes, ultimately became coal. This hypothesis goes by the name of the "drift theory." The other class of geologists contended, on the contrary, that coal had been formed in swamps and morasses, somewhat analogous to the formation of our peat bogs, and that, through the submergence of the land, these sites became inundated by the sea, and were afterwards covered up by sediments, as before described. This theory goes by the name of the "submergence theory." Many objections were formerly raised to it, the chief one being this :-that, if our coal beds were formed in this manner, in places where there are many seams piled one above another, and separated by masses of rock, there must have been continual submergences and upheavals, to allow the ancient vegetation to grow upon the surface. To this objection, however, we may answer, that it does not follow, that because there are several seams of coal piled one above the other, that there had been continual upheavals as well as submergences. All that is required is, that the submergences had been arrested from time to time, whilst the drifts from neighbouring lands were filling up the estuary, and, in course of time, thus rendering it sufficiently shallow for the growth of swampy vegetation. Both these classes of theorists, however, may be, and perhaps are, in the right; the error seems to have abided in their exclusive character, each contending that the theory of the other had no basis whatever. The submergence theory is now the most popular, and seems most naturally to

account for the preponderance of the phenomena which are observed in connection with the coal strata, without rejecting the facts adduced by the drift hypothesis. The following objections

are usually urged against the drift theory-one is, the freedom of coal generally from extraneous matter; had it been drifted it must have acquired portions of foreign substances in its transit, such as pebbles, gravel, &c. But since we find extensive seams of coal wholly free from any such matter, it is considered that those seams could not have been formed by drifted vegetation: another objection is, the general uniform thickness of each coal seam; one of the great northern coal fields extends over at least two hundred miles, and in the Lancashire coal field, there is a thin seam which extends from Blackburn to Whaley Bridge, a distance of more than 40 miles! Had the vegetation been washed down by either floods or torrents, such currents, either from the different specific gravities of portions of the vegetable mass, as roots, trunks, branches, leaves, &c., or from the mechanical obstructions which must have occurred during such a transit, would have deposited them in an unequal manner; whereas, as is well-known, no such appearances are met with in our coal seams. Again, the exceeding thinness of many of the coal seams, which often thin out into mere filaments, and extend in this condition over extensive areas, militates against the idea of any deposit of so thin a nature having been spread over such large spaces by the action of drifting. And further, the size of many of the coal seams, when the immense compression which they must have undergone is borne into consideration, is also an objection of a most insurmountable character. The beds of coal have been subjected to the pressure of over-lying rock, thousands of feet in vertical thickness, during a period of countless ages, so that the amount of pressure they have undergone is almost inconceivable, and the original quantity of vegetable matter must have been very great; but, according to the drift theory, it would be utterly impossible to transport such masses sufficiently requisite to form our great coal deposits, for, according to Professor Rogers, all the forests of the United States gathered into one heap, would fail to furnish materials equal to a single coal seam of that of Pittsburgh.

The high state of preservation in which many of the fossil plants. occur the perfect condition of the leaves-so that upon their venation a classification is founded, is also another proof of the quietness with which they have been deposited. The parts of