Earthquakes much more violent than now and volcanic outbursts often upset the steady order of things, but the earth was settling down. During this settling-down process rocks, as we have seen, were being formed by deposits; but they were very liable still to be invaded by bursts of volcanic activity from the inner cauldron of the earth, and they were very apt to be twisted out of their regular shape by great earth movements. They were also liable to be baked by the neighbourhood of the restless, unconfined molten rocks, nearer then to the surface than now. Geologists call the great period of time when all the rocks continually flowed out on to the surface of the earth, and were, in fact, all molten before they solidified, the Archæan Era (from a Greek word signifying the beginning). Next in order to these rocks are those which were laid down in the agitated times when the earth was still warm, and when the climate of the earth might be described as a continual thunderstorm. In this period earthquakes still had a great deal to do with the formation of the rocks, but then, as now, the sea and lakes and oceans laid them down. Geologists call this the Proterozoic Era. There are great masses of these Proterozoic rocks in North America. In Arizona the three periods of rock formation are sometimes visible together, and may, indeed, be perceived in some of our photographs; the Archæan all jumbled together being the lowest; Proterozoic lying crumpled or tilted over them, and the later rocks resting more regularly on these strata. In America, however, these separate ages of the Proterozoic rocks can be identified, and each age is represented by rocks thousands of feet in thickness. Three separate ages of rocks are found in this great era in North America. It is not very important to remember their names, which are merely those of the localities where these great deposits are most marked, but it is important not to forget that each of these depositions of rocks represents a period in the earth's history older than the lifetime of a river or a lake and as old as the lifetime of a continent. The lowest of these divisions consists of rocks that are much altered by the heat of the rocks below. The topmost division is hardly altered at all. In Scotland we have similar rocks. The Torridonian sandstones, 8000 to 10,000 feet thick, are believed to belong to this era. In France also, in Spain, Germany, Finland, Sweden, India, and Brazil, the Proterozoic rocks are found. In the lowermost of them are no signs that living things ever existed, but in the upper ones f life begin to appear. We may see in them to-day the first fossils. A fossil means literally a thing dug up, and was a term applied at first to all kinds of mineral substances taken out of the earth. We use the word now exclusively for the remains of plants and animals embedded in any kind of rock. In later chapters of this volume a good deal will have to be said