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.