The facts we have described all point to the conclusion that the presence of large quantities of water imprisoned in a mass of lava contributes greatly to its mobility. And this conclusion is supported by so many other considerations that it is now very generally accepted by geologists. The condition of this imprisoned water in lavas is one which demands further investigation at the hands of physicists. It has been suggested, with some show of reason, that the water may exist in the midst of the red-hot lava as minute particles in the curious 'spheroidal condition' of Boutigny, and that these flash into steam as the lava flows along. Lava, when extruded from a volcanic crater in a more or less completely fluid state, flows down the side of the cone, and then finds its way along any channel or valley that may lie in its course, obeying in its movements all the laws of fluid bodies. The lava-currents thus formed are sometimes of enormous dimensions, and may flood the whole country for many miles around the vent. Lava-streams have been described, which have flowed for a distance of from fifty to a hundred miles from their source, and which have had a breadth varying from ten to twenty miles. Some lava-streams have a thickness of 500 feet, or even more. These measures will give some idea of the enormous quantities of material brought from the earth's interior by volcanic action and distributed over its surface. The mass of lava which flowed out during an eruption off Reykjanes in Iceland, in the year 1783, has been calculated to be equal in bulk to Mont Blanc. There are many parts of the earth's surface, such as the Western Isles of Scotland and the North-east of Ireland, the Deccan of India, and large tracts in the Rocky Mountains, where successive lava-sheets have been piled upon one another to the height of several thousands of feet, and cover areas of many hundreds or even thousands of square miles. [Sidenote: FEATURES OF LAVA-STREAMS.] The more fusible basic lavas are as a general rule more liquid in character than any others, and it is these very liquid lavas that are usually found forming plateaux built up of successive lava-streams. The less liquid lavas, like those of Hungary and Bohemia, are not usually found flowing to such distances from the vent, but form dome-shaped mountain-masses.