The memorials of volcanic action remain, we had almost said permanently, among the decay of other rocks, though, of course, even hard volcanic or igneous rocks will be worn down in time. In many cases volcanoes themselves are left, though they may have been for ages extinct. In some volcanic regions where no great central cones have existed the vast floods of lava that were poured forth extend to-day as vast black plains of naked rock, mottled with shifting sand-hills, or as undulating tablelands carved by running water into valleys and ravines, between which the successive sheets of lava are exposed in terraced hills. Beyond the limits over which the lava sheets have spread there are often great veins or parapets or sunken walls of lava to which are given the names of igneous or volcanic "dykes." Dykes vary from less than a foot to one hundred feet in breadth, and often run in nearly straight courses sometimes for many miles. They consist usually of very hard rock like basalt, "andesite," or "diabase."
They were fissures in the earth's surface, and, after the manner we have described, the molten rock welled forth through these fissures, and spread out sheet after sheet, till like a rising lake it has not only overflowed the lower grounds but even buried all the minor hills.
Lava eruptions of this kind have taken place in recent years in Iceland. On a small scale they can be seen to take place in the island of Hawaii, where the outflows of lava reproduce for us, like models in miniature, the great outbreaks of the past. On the largest possible scale similar effects may be seen on the great lava plains of the Moon, where the giant craters that we can see through telescopes are not the mouths of extinct volcanoes, but the banked-up edges and shores of lava outflows.
On a much smaller scale than this, but still on a gigantic scale, the same thing took place in Western North America, where there are vast tracts of land which are best to be explained by supposing that there were once great outbreaks and overflows. The area which has there been flooded with lava has been estimated to be larger than that of France and Great Britain put together, and the depth of the total mass erupted reaches in some places as much as 3700 feet. Some rivers have cut gorges in this plain of lava, laying bare its component rocks to a depth of 700 feet or more. Along the walls of these ravines we see that the lava is arranged in parallel beds or sheets often not more than ten or twenty feet thick, each of which, of course, represents a separate outpouring of molten rock.
These are comparatively modern lava plains, although, of course, the outpourings in North America occurred ages before historic time, or, indeed, before there are any traces of man's existence on the earth. Such lava outflows can only occasionally be examined—as in the instance just quoted when rivers have cut deep into them. Consequently we have to speculate on the connection between the dykes and fissures and the lava flood itself. But in various parts of the world lava plains of much older date have been so eaten into and worn by the action of the elements that not only the successive sheets of lava are exposed but the rock floor over which they poured. Exposed also are the abundant dykes which served as the channels by which the lava rose to the surface. In Western Europe important examples of this structure occur from the north of Iceland through the inner Hebrides and the Faroe Islands to Iceland. This volcanic belt presents a succession of lava sheets, which even yet, in spite of enormous waste, are in some places more than 3000 feet thick. These sheets are nearly flat and rise in terraces over one another into green grassy hills or into the dark fronts of lofty sea-washed precipices. Where sheets have been stripped off or worn down by wind and weather thousands of volcanic dykes are exposed. These dykes are, as it were, the roots of which the lava sheets were the branches; and even where the whole of the material that gushed up to the surface has been worn away the dykes remain as evidence of the vigour and energy of the volcanic forces.
CHAPTER X
THE EARTH AS THE ABODE OF LIFE
In the last chapter we spoke of the formation of the atmosphere of the earth and of the growth of the oceans. We must now consider the formation of these more closely, and we must distinguish between the great vaporous clouds which rolled about the earth in its molten state and the settled atmosphere which formed about it when it grew cooler.
After the earth had begun to solidify it was at first covered with a collection of porous fragments of rock covering the earth like a shell and containing the elements of water. Such materials in general appearance would be not unlike the pumice stone which is expelled from volcanoes to-day. Those who have never had the fortune to see volcanic eruptions for themselves usually imagine that the volcano throws out nothing but fire and smoke. As a matter of fact it throws out vast quantities of vapour, of which, according to Sir Archibald Geikie, 999 parts in 1000 are steam. At the great eruption of Mount Pelée the cloud of steam continually arising from the volcano for months in succession was several cubic miles in measurement. Consequently it will be seen that the porous volcanic rocks with which the young earth was covered contained all the materials for water-manufacture within themselves. As the water began to form, squeezed out of the porous rocks as we can squeeze it out of a sponge (or as we might steam it out if we put the moist sponge in an oven), it gathered itself into reservoirs underground. As it increased in bulk it rose nearer to the surface; because, of course, owing to the heat of the inner portions of the earth it could never succeed in sinking below a certain depth. Doubtless it first appeared at the bottom of the pits which had been sunk by volcanoes or volcanic action. There must have been innumerable depressions in the earth's surface as widespread and deeper than those which we can perceive on the rugged surface of the Moon. We may gain an idea on a very minute scale of what the first pits of water were like from the examples (formed, however, at a much later period and probably in a different way) of the crater lakes that are left to-day. Some curious examples of "crater lakes" are to be found in the Eifel district of Germany, an ancient volcanic region which lies in the triangle formed by the junction of the Rhine and the Moselle at Coblenz. One of the pleasing peculiarities of this district is that, owing to the volcanic nature of the soil, the neighbourhood is seldom dusty, even in August or September, after the dry continental summer. It is well worth visiting for its castles as well as for its crater lakes and other volcanic relics, and it is the scene of R. L. Stevenson's romance Prince Otto. The chief crater lakes are between Daun and Manderscheid. There are, of course, many other and larger crater lakes in existence, but we select these because they are so easily accessible.
The flow of the lakes into one another followed. Innumerable lakelets developed into rivers or chains of lakes on the surface of the young planet, continually becoming larger bodies of water, till they developed into the vast irregular oceans of to-day. This evolution is of great importance from a geological point of view, because it leads the way to the origin of the ocean basins and the great platforms of land which we call continents. It is easy to see that because of the weight of water in the depressions the earth under the waters tended to become more and more depressed, so that the water areas tended to grow larger and deeper. The wash of earths from the land tended to build its borders out into the water basins, but the deepening and spreading of the water basins is believed to have been the most marked feature of the earth's early growth. All this time the earth was growing in diameter and circumference.[9] When this growth ceased other causes and effects came into play, and the proportions of sea and land became better balanced.