Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER II AGENTS OF DENUDATION CHEMICAL COMPOSITION OF ROCKS—EPICENE AGENTS—INSOLA- TION AND DEFLATION—CHEMICAL AND MECHANICAL ACTION OF RAIN ACTION OF FROST ; OF PLANTS AND ANIMALS | OF UNDERGROUND WATER ; OF BROOKS AND RIVERS—RATE OF DENUDATION—DENUDATION AND SEDIMENTATION GO HAND IN HAND. THE present, geologists tell us, contains the key to the past. If we wish to find out how rocks have been removed, and what has since become of them, we must observe what is taking place under the influence of existing agents of change. How, then, are rocks being affected at present ? We do not proceed far in our investigation before we discover that they are everywhere becoming disintegrated. In one place they are breaking up into angular fragments ; in another, crumbling down into grit, sand, or clay. Brooks and rivers and the waves upon our coasts are constantly undermining them ; everywhere, in short, rocks are being assaulted and reduced. But in order to bring this fact more forcibly before the reader, it will be well to sketch, as briefly as may be, the general character of the warfare which is being waged againstrocks over all the land-surface, and to note the various results that flow from this incessant energy of the epigene or superficial agents of change. As these agents are often associated in their work, it is sometimes hard, or even impossible, to say which has played the most effective part in the demolition of rocks. Nevertheless, it will conduce to clearness if we endeavour to consider the operation of each by itself, so far, at least, as that is possible. Before doing so, however, we must glance for a moment at the general characters of rocks. We have already taken note of the fact that rocks are of various origin —igneous, derivative, and metamorphic. It is...