But the interesting point is, that true coal-beds, with similar fossils to those of our own coal-measures, are found at Spitzbergen and at Bear Island in East Siberia, both far within the Arctic Circle, again indicating a great uniformity of climate, and probably a denser and more vapour-laden atmosphere, which would act as a blanket over the earth and preserve the heat brought to the Arctic seas by the ocean currents from the warmer regions.
The still earlier Silurian rocks are also found abundantly in the Arctic regions, but their fossils are entirely of marine animals. Yet they show the same phenomena as regards climate, since the corals and cephalopodous mollusca found in the Arctic beds closely resemble those of all other parts of the earth.[16]
Many other facts indicate that throughout the enormous periods required for the development of the varied forms of life upon the earth, the great phenomena of nature were but little different from those that prevail in our own times. The slow and gentle processes by which the various vegetable and animal remains were preserved are shown by the perfect state in which many of the fossils exist. Often trunks of trees, cycads, and tree-ferns are found standing erect, with their roots still imbedded in the soil they grew in. Large leaves of poplars, maples, oaks, and other trees are often preserved in as perfect a state as if gathered by a botanist and dried between paper for his herbarium, and the same is especially the case with the beautiful ferns of the Permian and Carboniferous periods. Throughout these and most other formations well-preserved ripple-marks are found in the solidified mud or sand of old seashores, differing in no respect from similar marks to be found on almost every coast to-day. Equally interesting are the marks of rain-drops preserved in the rocks of almost all ages. Sir Charles Lyell has given illustrations of recent impressions of rain-drops on the extensive mud-flats of Nova Scotia, and also an illustration of rain-drops on a slab of shale from the carboniferous formation of the same country; and the two are as much alike as the prints of two different showers a few days apart. The general size and form of the drops are almost identical, and imply a great similarity in the general atmospheric conditions.
We must not forget that this presence of rain throughout geological time implies, as we have seen in our last chapter, a constant and universal distribution of atmospheric dust. The two chief sources of this dust—the total quantity of which in the atmosphere must be enormous—are volcanoes and deserts, and we are therefore sure that these two great natural phenomena have always been present. Of volcanoes we have ample independent evidence in the presence of lavas and volcanic ashes, as well as actual stumps or cores of old volcanoes, through all geological formations; and we can have little doubt that deserts also were present, though perhaps not always so extensive as they are now. It is a very suggestive fact that these two phenomena, usually held to be blots on the fair face of nature, and even to be opposed to belief in a beneficent Creator, should now be proved to be really essential to the earth's habitability.
Notwithstanding this prevalence of warm and uniform conditions, there is also evidence of considerable changes of climate; and at two periods—in the Eocene and in the remote Permian—there are even indications of ice-action, so that some geologists believe that there were then actual glacial epochs. But it seems more probable that they imply only local glaciation, owing to there having been high land and other suitable conditions for the production of glaciers in certain areas.
The whole bearing of the geological evidence indicates the wonderful continuity of conditions favourable for life, and for the most part of climatal conditions more favourable than those now prevailing, since a larger extent of land towards the North Pole was available for an abundant vegetation, and in all probability for an equally abundant animal life. We know, too, that there was never any total break in life-development; no epoch of such lowering or raising of temperature as to destroy all life; no such general subsidence as to submerge the whole land-surface. Although the geological record is in parts very imperfect, yet it is, on the whole, wonderfully complete; and it presents to our view a continuous progress, from simple to complex, from lower to higher. Type after type becomes highly specialised in adaptation to local or climatal conditions, and then dies out, giving room for some other type to arise and be specialised in harmony with the changed conditions. The general character of the inorganic change appears to have been from more insular to more continental conditions, accompanied by a change from more uniform to less uniform climates, from an almost sub-tropical warmth and moisture, extending up to the Arctic Circle, to that diversity of tropical, temperate, and cold areas, capable of supporting the greatest possible variety in the forms of life, and which seems especially adapted to stimulate mankind to civilisation and social development by means of the necessary struggle against, and utilisation of, the various forces of nature.
Water, its Amount and Distribution
on the Earth
Although it is generally known that the oceans occupy more than two-thirds of the whole surface of the globe, the enormous bulk of the water in proportion to the land that rises above its surface is hardly ever appreciated. But as this is a matter of the greatest importance, both as regards the geological history of the globe and the special subject we are here discussing, it will be necessary to enter into some details in regard to it.
According to the best recent estimates, the land area of the globe is 0.28 of the whole surface, and the water area 0.72. But the mean height of the land above the sea-level is found to be 2250 feet, while the mean depth of the seas and oceans is 13,860 feet; so that though the water area is two and a half times that of the land, the mean depth of the water is more than six times the mean height of the land. This is, of course, due to the fact that lowlands occupy most of the land-area, the plateaus and high mountains a comparatively small portion of it; while, though the greatest depths of the oceans about equal the greatest heights of the mountains, yet over enormous areas the oceans are deep enough to submerge all the mountains of Europe and temperate North America, except the extreme summits of one or two of them. Hence it follows that the bulk of the oceans, even omitting all the shallow seas, is more than thirteen times that of the land above sea-level; and if all the land-surface and ocean-floors were reduced to one level, that is, if the solid mass of the globe were a true oblate spheroid, the whole would be covered with water about two miles deep. The diagram here given will render this more intelligible and will serve to illustrate what follows.