By such a method of examination we may frame for ourselves pictures of the earth's surface long before history began, or before man roamed the earth. It is for this reason that geology has been called the science that investigates the history of the earth. The records in which this history is chronicled are the soils and rocks underneath our feet. It is the task of the geologist so to arrange and interpret these records as to show through what successive changes the globe has passed, and how the dry land came to wear the aspect which it presents at the present time. To do this efficiently the geologist has to learn many things. He has to observe very closely the changes which are going on about him on the world's surface. Only in so far as he makes himself acquainted with these sudden changes can he hope to follow intelligently and successfully the story of earlier phases in the earth's progress. Nor is it sufficient to observe, however closely, inanimate things. If he did not know the peculiarities of fresh-water shells, how would he be able to say the shells in the marl deposit were fresh-water animals (and that therefore a lake once lay there) and not sea shells. If the labour of the geologist were concerned merely with the former changes of the earth's surface--how sea and land have changed places, how rivers have altered their courses, how valleys have been dug out, and how mountains have been carved, how plains have been spread out, and how all these things have been written on the framework of the earth--he would still feel one very great want, the want of living interest. But that also his science gives him, for in these past eras living things dwelt and moved and had their being. And it is one of the most entrancing pursuits of the geologist to trace their lives, their descent and ascent, and the relics of themselves that they left. CHAPTER III EFFECTS OF WEATHER ON THE EARTH'S HISTORY