What I have just said is about all that we know of the formation of the oldest and hardest granite rocks: but there is something going on now which confirms the belief that the materials of which they are made were melted together by a greater heat than we can make in our furnaces for melting iron; for I should tell you that it is easier to melt iron and copper than granite rocks. Volcanoes often throw out melted earths which when cooled appear to be made of much the same materials as these granites. SANDSTONE. But we know more of the manner of the formation of sandstone. This rock is composed of rounded grains of sand just like that we find upon the sea shore. If you take a handful of this sand and squeeze it tightly, it will keep together a little while. Now suppose a quantity of this sand was pressed by a very great weight—the weight of a large hill for instance—after many years the grains would stick firmly together, and become a sort of stone. It is in this way the sandstones must have been formed, and perhaps heat helped the work, though not so great a heat as melted the granite. The sand, after it had been washed upon the sea shore, became gradually covered with other earths hundreds of feet thick, and the immense weight above it pressed it into stone: but you may imagine how very long a time it took to do this. Sandstones are used for building, but they do not last very long; the frost makes little cracks in them and they soon crumble away to the grains of sand of which they were made. Several fossils are found in some of these sandstones, which have been formed at many different periods of the earth’s history. CHALK. You have seen those high cliffs of chalk along the south coast of England, perhaps, and you have wondered what that beautiful white earth was, and how it came there. It is found in many parts of the world, and the south and south-east of England are to a great extent composed of it. The material is called by chemists carbonate of lime. It is almost entirely made up of minute shells called _foraminifera_, from two Latin words which mean that there are many openings or chambers in their shells, and there are many beautiful fossils called _ammonites_ imbedded in the chalk. These are shell-fish, two or three inches, and sometimes a foot across, and their shape is very like that of the young leaves of the common fern before it has opened in the spring. Millions of these tiny foraminifera are living now in parts of the Atlantic and Pacific oceans, and when they die their shells sink to the bottom and form a greyish mud, something like chalk. When H.M.S. “Challenger” was sent out in the year 1873, to find out what was at the bottom of the deepest seas of the World, great interest was felt in the expedition, because we were sure that we should learn something about the manner in which some of the rocks were formed.