“I read that this stratum of rock was once a beach of sand. It was just such sand as a child likes to play in; just such sand as it is fabled that the Sibyl took in her hands when she asked long life of Apollo.” “And how did it become rock?” “You must know that all new rocks are made of the wear and tear of older rocks, reconstituted and relaid. This sandstone was once sand, and the sand was chiefly tiny quartz crystals, the residue left after the softer mineral portions of certain ground-up older rocks had been carried away. These quartz crystals could not be melted, nor held in solution by water; being heavy they could not be held in suspension by water. So the water which was carrying about the rubbish of the older rocks, dropped the sand, and it formed long beaches, such as we see to-day. Observe that sand seems to be more plentiful than any other substance in the earth’s crust. Most soil is largely composed of it. If it were not for the sand in the soil to keep its particles loose and allow the percolation of water, and the growth of roots, the soil would soon become packed so heavily that almost nothing could grow. The sea-shore, the river-beds, the great Sahara, are sand, sand, sand. As sand is the most plentiful loose material in the earth now, so no doubt it has always been, and we find that sandstone, namely, rock made of sand, is the most abundant in the structure of the globe. The grains of quartz themselves are naturally gray or white, but they are covered with a film or crust which takes different hues from coloring matter mixed with it. In iron regions the sandstone is red, and in anthracite coal regions the sandstone is dark gray or nearly black. Other sandstones are brown, yellow, or purple, according as they are mixed with other coloring matter. “But you asked me how the loose sand became rock. Understand that the most important factor in the process was an indefinitely long time, while the agents required were very simple--great pressure and the presence of water. The water carrying with it silica, or some kindred material, formed a cement about each grain, and thus aided by pressure and a certain amount of heat--for in this world nothing is done without heat--the individual grains were finally consolidated into massive rock.” “What else have you read?” asked other pupils, as they too began earnestly to contemplate the ledge of sandstone. “That was only the preface, or first chapter. I remember that the grains of sand, composed largely of silica or tiny quartz crystals, were heaped up here by water, which washed from the crystals and then carried away, little particles of lighter matter, as mud. When I look at these long, gently sloping ledges of sandstone, I read that they were once a beach. The ravine and the hills, the cornfields and the forests are now here, but once--the sea. Warm salt waters ebbed and flowed here, and shells were rolled up on this beach and were carried out again by the tides.”