a thousandfold. Great Britain, Belgium, and North America possess some of the most valuable coal measures yet known, and the annual yield from these three sources alone must be enormous. Dr. Siemens has calculated that by the end of this century we shall require a yearly supply of 250,000,000 tons, basing his estimate on the report of a Royal Commission, which stated the production from the mines of Great Britain to be 123,000,000 tons in the year 1872, which had risen to more than 134,179,000 tons in 1877. Extending these figures to the coal-fields of the whole earth, we have a faint conception of the vast accumulations they represent, due to the decomposition of atmospheric air by plant life.

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When George Stephenson was asked what was the ultimate cause of motion in his locomotive engine, he replied, in graphic, but not strictly philosophical terms, that it went by the bottled-up rays of the sun." Coal is, no doubt, the product of solar energy acting upon the surface of our earth, and Dr. Siemens has thus aptly described the physical and chemical process. "The rays of the sun represent energy in the form of heat and light, which is communicated to our earth through the medium which must necessarily fill the space between us and our great luminary. If these rays fall upon the growing plant, their effect disappears from direct recognition by our senses, inasmuch as the leaf does not become heated, as it would if it were made of iron or dead wood; but we find a chemical result accomplished, viz., carbonic acid gas, which has been

absorbed by the leaf of the tree from the atmosphere, is there dissociated, or separated into its elements, carbon and oxygen-the oxygen being returned to the atmosphere, and the carbon retained to form the solid substance of the tree."

Whatever, then, the measure of solar energy conveyed to the plant in the formation of one pound of carbon, as represented by wood fibre, that energy will reappear when the coal is heated sufficiently to enable its carbon to combine with the oxygen of the air, and restore to it the carbonic acid originally abstracted. The plant, during its life, actually breaks up the connection between the oxygen and carbon contained in the carbonic acid, restoring the former to the air and appropriating the latter to itself. Under the influence of fire this process is merely reversed-oxygen being taken from the air to combine with the carbon of the plant, which is then "consumed," as we say, but has, in fact, been reconverted into its original constituents, leaving behind nothing but a small quantity of mineral ash-generally lime, silica, and iron-taken up by the plant tissues mainly from the soil.

When chemically analyzed, coal is found wanting in some of the constituents of wood fibre; but this does not invalidate the conclusion, founded on other strong evidence, that it is derived in the first instance from vegetation; and, as in the case of other rocks, we cannot trace all the influences which have been brought to bear upon it, nor expect to be able to reconstruct the original plants in their entirety. Sufficient, however,

remains, both in the plant forms actually preserved, and in the more or less pure carbon invariably present in true coal, to banish all doubt of its organic origin.

In the limestones of the Carboniferous system, the carbon is in combination with oxygen and lime, and these rocks are mainly of animal origin; while in the coal itself the carbon greatly predominates.

The lower orders of the animal and vegetable kingdoms have been chiefly concerned in building up the vast masses of rock constituting the organic series—in the aggregate tens of thousands of feet thick; while the giants of the earth, the highly organized animals and plants, have done comparatively little. We have seen what coral animals and minute foraminifera can do ; not less stupendous is the work of another foraminifer, the Nummulite, also an inhabitant of the sea. The shell is in shape similar to a coin (whence the name, Latin, nummus), and about the size of a farthing, divided internally into numerous chambers. The general appearance of a nummulitic limestone is that of a mass of coins lying in a heap. This rock rises in mountain ranges from Tibet, passing through the Himalayas to Persia and Egypt, and appears in the Carpathians, the Alps, the south of France, the Pyrenees, Spain, Algeria, and Morocco. In Tibet, the formation reaches 16,000 feet above the level of the sea, and is in many places thousands of feet thick. That wonderful example of human skill and labour, the Great Pyramid, is built entirely of the rock. Yet this limestone is a comparatively recent formation, certainly not earlier

than the beginning of the TERTIARY era; until which time the vast shelly accumulation, now lifted among the clouds, was quietly reposing on the bed of its parent

ocean.

In the chapters devoted to Paleontology, the rocks will be considered in the order of their succession, and with reference to the progressive development of life from lower to higher types of organization; but, as a basis for this, it was advisable first to sketch the framework of the structure which has been the home of living beings during innumerable ages.

CHAPTER VI.

THE GLACIAL PERIODS.

Climate of the Arctic regions in past geological times compared with the present-Proofs of its former temperate character in the extinct fauna and flora of the SECONDARY and TERTIARY eras-Former Arctic condition of European climate-Considerations which led to the theory of Glacial action-The physical properties of ice, as exemplified in Alpine and Greenland glaciers and icebergs of the polar seas-Contemporary glaciation of Arctic and Antarctic continents, the Himalayas, etc.-Extension of Glacial deposits to the neighbourhood of London-Astronomical and other theories in explanation of climatic variations-Evidences of the existence of man in Glacial times-Retrospect.

AT the present moment, a very large area in North America and Asia and the intervening islands is subject to a climate so severe that, wherever the elevation of the land is at all considerable, the ice and snow covering it are permanent at all seasons, or only partially melted in summer. In the Antarctic regions these conditions appear to be greatly intensified-judging from the extension of ice northwards in the Southern Ocean, which, while it prevents any satisfactory exploration of the land towards the South Pole, indicates the prevalence of a greater degree of cold than we have experience of in the Northern hemisphere. Without the aid of geology,