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.