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coral, several genera of shell-fish, some trilobites, the gigantic crustaceans, pterygotus and stylonurus, the cephalaspis, pteraspis, coccosteus, pterichthys, and other fishes, have never been detected beyond the limits of the Old Eed formation. They came in during the system, and died out before its close; thus implying not only long lapses of growth, and reproduction, and decay, but an onward march in that creative process by which the world has been peopled by different and higher races during the advancing periods of its geological history. How wonderful this newer knowledge of life which geology imparts! how marvellous the ever-ascending yet never-completed scheme of vitality it reveals! To our forefathers the life of the present era was but a repetition of the life of former ages; to us the life of the present is but a passing aspect, differing from the thousand aspects that went before, but inseparably bound up with them in one great scheme of ever-varying yet everwidening development.
Such once more is the Old Eed Sandstone, a system that owes its interest much more to its scientific than to its economic importance. Indeed, with the exception of building-stones used for local purposes, some indifferent limestones, and paving-flags, such as those of Caithness and Forfar, there are no rocks of any commercial value among its strata; and the only accidental minerals we are aware of are occasional poorish veins of galena, veins of baryta, salt-springs like those of North America, and the pebbles of agate, carnelian, and the like (Scotch pebbles), obtained from the amygdaloidal trap-rocks that traverse the system. Its chief interest centres round its fossil fishes and crustacea, subjects rendered popular now nearly thirty years ago by the writings of Hugh Miller and Agassiz, and still attracting attention by the newer forms that are year after year made known by the labours of younger geologists.* And surely what geologists are labouring to reveal, the man of ordinary intelligence may make some effort to comprehend and enjoy. It must indeed be a dull and incurious mind that cannot be induced to take an interest in the history of the world he inhabits, and to trace in its formations the record of operations which took place, and the nature of beings that lived and died, thousands of ages before the human race was created to become participators in the same ever-varying and ever-advancing scheme of vitality. Astronomy may be a loftier theme, but the loftiness of its topics only renders them the colder and more remote. Geology, on the other hand, has ever an immediate and human interest. The Earth's Past is inseparably interwoven with her Present; that which now lives is intimately associated in plan and relationship with that which lies fossil in the rocks beneath us; this plan has been steadily evolving during untold ages; man's own history is but part and parcel of that plan; and surely whatever tends to exalt our conceptions of creation can never tend to weaken our reverence for the power, wisdom, and goodness by which it is directed and sustained.
* We allude in particular to the labours of the late Professor Pander among the Old Red fishes of Russia; the numerous discoveries of new crustaceans and fishes in the flagstones of Forfarshire by Mr Powrie of Reswallie; and the long-continued observations of Dr Gordon and others among the sandstones of Moray and Ross-shire.
COAL AND COAL-FOEMATIONS.
COAL, ITS ORIGIN AND FORMATION—MINERALISED VEGETATION—
There is no mineral in the crust of the earth more essential
our main object will be to exhibit the points upon which geologists are generally agreed, rather than to distract the non-scientific reader with the minutiae upon which some of them still continue to differ.
What is coal 1 is a question more satisfactorily answered by a little roundabout explanation than by a direct reply. To say that coal is altered and mineralised vegetable matter is true; but the definition is too curt to be readily intelligible. Every one knows something of peat and peat-mosses; well, this peat is simply coal in its first stage of development. Were the peat-moss submerged and covered over by deposits of mud and clay and sand, it would in course of time undergo important chemical changes, by which part of its gaseous contents (oxygen, hydrogen, &c.) would be discharged, and the mass reduced to a compact coaly substance known as lignite or brown-coal. Such browncoals are abundant in many countries (Germany, Austria, New Zealand, &c), and worked for economical purposes; and were these subjected to still further changes they would, in course of ages, become converted into shining stony coals like those which are now raised so largely from the coalfields of Great Britain. The truth is, coal occurs in the earth's crust in every stage of development, from the peatmosses and swamp-growths still in process of accumulation on the surface, down through the tertiary brown-coals to the bituminous stone-coals of the secondary and primary periods, and from these again down to the still older nonbituminous anthracites and graphites. All, in fact, have had a similar origin. They are mere vegetable masses that have undergone different degrees of mineralisation—the recent vegetable full of volatile matters, the lignites less so, the bituminous coals giving off smoke and flame, the anthracites barely smoking, and the graphites masses of pure debituminised carbon. They are all coals, and belong to the same family—those in the younger formations still retaining much of their vegetable structure and full of volatile matter, while those in the older formations have seemingly lost all traces of structure, and, through chemical changes, have been all but deprived of their volatile constituents. But even where no structure is obvious to the naked eye, it can generally be rendered apparent by submitting thin transparent slices to the microscope. By this means the vegetable origin of the most compact and glistening coal is often revealed as clearly as the tissues in living plants, and thus the observer is enabled to determine not only the organic nature of the mass, but the botanical peculiarities of the order concerned in its formation.
Since coal is thus merely altered and mineralised vegetable matter, and since vegetation must have, flourished more or less during every period of the earth's history, there must be coals of some kind or other occurring in every geological formation. It may appear more abundantly and more availably in one formation than in another, and may be present in some latitudes while it is absent from others; still, believing in the uniformity of nature's operations, we must be prepared to admit its presence in every stratified system, and not to regard it, as was at one time done, as a product peculiar to the Carboniferous era. Arranging the formations in chronological order, we have their coals, or rather the coal family, associated with them in something like the following conditions:—
Cretaceous, Lignites and Coals.
New Red Sandstone, Coals.
Carboniferous, Coals and Anthracites.
Old Red Sandstone, Coals and A nlhracites.