Silurian, Laurentian, .Anthracites. It is true that in the older formations we have but a very scanty exhibition of coaly substances, and it is equally true that hitherto the most extensive developments have been found in strata of Carboniferous age; but it is nevertheless the fact that coal-fields of great value occur in the oolitic and cretaceous rocks, and that brown-coals are common in almost every tertiary district. It may render the subject more intelligible and attractive if we take the formations seriatim-beginning with the recent and apparent, and working down through the older and more obscure. The coals of the present day are the peat-mosses, the swamp-growths, and the vegetable drifts borne down by rivers and deposited in their estuaries. We have no means of ascertaining the extent or thickness of vegetable drifts, though some, like the "Rafts" of the Mississippi, are of considerable thickness and extent; but we know that large areas in all the temperate and colder latitudes are occupied by peat-mosses and swamp-growths-the lake region of North America, Canada, the Southern States, Siberia, Northern Europe, Denmark, Holland, and our own islands. We have no reliable statistics of the extent and thickness of peat-mosses either in Europe, Northern Asia, or North America; but in the recently-published Reports on the Geology of Canada by Sir William Logan, a number of details are given, from which we learn that upwards of 300 square miles of that country are occupied by peat-mosses varying from 3 to 30 feet in thickness. If such be an approximation to the amount of peaty surface in the surveyed portion of Canada, the amount in the whole of British North America, Northern Europe, and Northern Asia, must be something enormous. These peat-beds are often of great thickness, and date from the growth of the current year to the very dawn of the Quaternary epoch; loose and turfy above, firm and peaty a few feet down, and at greater depths black and dense as some varieties of lignite. Indeed we have seen varieties of Dutch peat taken at 30 feet deep indistinguishable from some lignites; and all that seems necessary to convert them into true brown-coals are the cover and pressure of superincumbent strata, and time sufficient to effect those further chemical changes to which lignites and brown-coals have been generally subjected. We see, therefore, in the compressed vegetable matter we call peat, and which has been formed by the growth and decay of certain plants* during many centuries, the first stages of coal; and when we come to consider the older formations, we shall find that many of their coal-seams have had a similar origin. And just as this peat is sometimes earthy and mingled with stony matter that has been washed into the swamps and hollows by rains and rivers, so we may expect some of the old coals to contain similar impurities, and to be less valuable as fuel. The next and older series of coals embraces the lignites or wood-coals, the brown-coals, and board-coals of the Tertiary strata. As these names imply, their woody or vegetable texture is still apparent, and they are generally of a brown or earthy hue, compared with the black and glistening lustre of the coals of the older formations. Alternating with clays, marls, sands, and gravels, they have evidently been formed partly in fresh-water lakes and swamps, and partly in areas that have been submerged and covered over by marine deposits. In some instances they are earthy, and composed of the drifted trunks and branches of trees; * See Sketch entitled "Recent Formations." and in others the submerged and fallen forest-growth can be traced as clearly as it can be in some of the shallower peat-beds of Scotland. Most of these lignites, whether as once worked at Bovey in Devonshire, or as still worked in Germany, Prussia, Austria, New Zealand, and other countries, may be described as coal in its second stage of consolidation and mineralisation. In the mine they are soft, full of water, and easily cut; and when brought to the surface, dry and break up, and soon crumble down under the influence of the weather. They are also less regular in their bedding than the older coals-thickening and thinning capriciously; but in some instances their bedding is regular and continuous over considerable areas, and their quality is so much improved that they are scarcely distinguishable from ordinary coal. One remarkable instance of this kind, the Zsil valley in Translyvania, was visited in 1862 by Professor Ansted, who found not lignite, but coal differing little from some varieties of English coal, lying in regular beds of great thickness, and alternating with shales, ironstones, and grits. Of course all the vegetable accumulations of the Tertiary system are not precisely of the same age, nor have they been deposited under the same conditions, and thus we may expect to find differences among them, just as among the coals of the older formations. And hence it happens that some of these lignites are scarcely fit for pottery or brick-kiln purposes, while others (certain compact and lustrous varieties like those of Transylvania) are advantageously used for locomotive engines and for metallurgical operations. Although seams of lignite are occasionally found in the Cretaceous and Oolitic systems, yet, generally speaking, the Secondary strata—the Chalks, Oolites, and New Red Sandstones-are characterised by the presence of true coals. The seams may not be continuous over extensive areasthat is, may thicken and thin somewhat capriciously-but still mineralisation of the mass is complete, and we are presented with bituminous coals of varying commercial value. Such Secondary coal-fields occur at Brora and Whitby in Britain; at Fünfkirchen and Oravicza in Austria; at Burdwan, Nerbudda, and other districts in India; in Burma, Borneo, and Labuan; in New Zealand; Natal in South Africa; Vancouver Island and British Columbia; at Richmond in Virginia; and in all likelihood in other districts that have not yet been sufficiently sur veyed. These coals, so far as they have been geologically examined, have been accumulated precisely like the peatmosses, swamp-growths, and vegetable-drifts of the present day. Some have evidently grown in situ, and accumulated. in great thickness and purity for ages; in others the growth has been interrupted by overflowings of the water, and earthy layers are not unfrequent in the mass; while in others, again, the mass is so irregular in thickness and composition, that it at once recalls the idea of drift and heterogeneous deposition. Whatever the thickness or composition, they are true bituminous coals, thus disproving the belief which was generally entertained some thirty years ago, that all true coal was a product of one geological epoch only, and necessarily belonged to the Carboniferous formation. On this point we can offer nothing more convincing than the following extract from Sir Charles Lyell's description of the Richmond coal-field in Virginia: These Virginian coal-measures are composed of grits, sandstones, and shales, exactly resembling those of older or primary date in America and Europe, and they rival, or even surpass, the latter in the richness and thickness of the coal-seams. One of these the main seam-is in some places from 30 to 40 feet thick, com 66 posed of pure bituminous coal. On descending a shaft, 800 feet deep, in the Blackheath mines in Chesterfield county, I found myself in a chamber more than 40 feet high, caused by the removal of the coal. Timber props, of great strength, supported the roof; but they were seen to bend under the incumbent weights. The coal is like the finest kinds shipped at Newcastle, and when analysed yields the same proportions of carbon and hydrogen fact worthy of notice when we consider that this fuel has been derived from an assemblage of plants very distinct specifically, and in part generically, from those which have contributed to the formation of the ancient or paleozoic coal." a "It is true, however," says one of the most experienced and practical of British geologists (Professor Ansted), "that the great coal- fields of England, of Belgium, of Spain, of France, and of North America, besides those of Bohemia, Moravia, and the Rhine, of Russia and China, and probably of Australia, belong to the oldest or paleozoic rocks; and that for some reason that may perhaps be better understood at a future time than it now is, these deposits are more regular, more uniform over large areas, and in that sense more to be depended upon, than those of newer date."* In other words, they belong to the Carboniferous system, that great series of limestones, sandstones, shales, * There can be no doubt that the difference here alluded to has arisen, partly from the peculiar distribution of sea and land during the Carboniferous era, which permitted over extensive areas a moist, genial, and equable climate, and partly from the peculiar character of the vegetation of the period, which seems to have been at once of rapid growth and of a kind eminently fitted for preservation. Physical conditions like a moist, genial, and equable climate may recur in the course of nature, but the Life of each geological system is peculiar, and vanishes with the period to which it belongs. The Carboniferous flora disappeared with its epoch; and no flora equally fitted for the formation of coal has since recurred, or may ever again recur, in the progressional course of creation. |