LIST OF PLATES.

EARTH'S ORBIT WHEN ECCENTRICITY IS AT ITS SUPERIOR

LIMIT

Frontispiece.

PLATE

I. SHOWING AGREEMENT BETWEEN THE SYSTEM OF OCEAN-
CURRENTS AND WINDS

To face page 212

VII. MAP OF THE MIDLAND VALLEY (SCOTLAND), SHOWING

BURIED RIVER CHANNELS

219

222

313

449

453

,, 221, line 3 from bottom, for "temperature" read "tempera

tures."

,, 222, Plate, for "Lat. 70° S." read Lat. "20° S."

,, 327, line 14, for "?" read "!"

,, 385, line 14, for "730" read “720.”

CHAPTER I.

INTRODUCTION.

The Fundamental Problem of Geology.-Geology a Dynamical Science.-The Nature of a Geological Principle.-Theories of Geological Climate.-Geological Climate dependent on Astronomical Causes.-An Important Consideration overlooked.-Abstract of the Line of Argument pursued in the Volume.

The Fundamental Problem of Geology.-The investigation of the successive changes and modifications which the earth's crust has undergone during past ages is the province of geology. It will be at once admitted that an acquaintance with the agencies by means of which those successive changes and modifications were effected, is of paramount importance to the geologist. What, then, are those agencies ? Although volcanic and other subterranean eruptions, earthquakes, upheavals, and subsidences of the land have taken place in all ages, yet no truth is now better established than that it is not by these convulsions and cataclysms of nature that those great changes were effected. It was rather by the ordinary agencies that we see every day at work around us, such as rain, rivers, heat and cold, frost and snow. The valleys were not produced by violent dislocations, nor the hills by sudden upheavals, but were actually carved out of the solid rock, silently and gently, by the agencies to which we have referred. "The tools," to quote the words of Professor Geikie, "by which this great work has been done are of the simplest and most every-day order-the air, rain, frosts, springs, brooks, rivers, glaciers, icebergs, and the sea. These tools have been at work from the earliest times of which any geological record has been preserved. Indeed, it is out of

the accumulated chips and dust which they have made, afterwards hardened into solid rock and upheaved, that the very framework of our continents has been formed." *

It will be observed-and this is the point requiring particular attention—that the agencies referred to are the ordinary meteorological or climatic agencies. In fact, it is these agencies which constitute climate. The various peculiarities or modifications of climate result from a preponderance of one or more of these agencies over the rest. When heat, for example, predominates, we have a hot or tropical climate. When cold and frost predominate, we have a rigorous or arctic climate. With moisture in excess, we have a damp and rainy climate; and so on. But this is not all. These climatic agencies are not only the factors which carved out the rocky face of the globe into hill and dale, and spread over the whole a mantle of soil; but by them are determined the character of the flora and fauna which exist on that soil. The flora and fauna of a district are determined mainly by the character of the climate, and not by the nature of the soil, or the conformation of the ground. It is from difference of climate that tropical life differs so much from arctic, and both these from the life of temperate regions. It is climate, and climate alone, that causes the orange and the vine to blossom, and the olive to flourish, in the south, but denies them to the north, of Europe. It is climate, and climate alone, that enables the forest tree to grow on the plain, but not on the mountain top; that causes wheat and barley to flourish on the mainland of Scotland, but not on the steppes of Siberia.

Again, if we compare flat countries with mountainous, highlands with lowlands, or islands with continents, we shall find that difference of climatic conditions is the chief reason why life in the one differs so much from life in the other. And if we turn to the sea we find that organic life is there as much under the domain of climate as on the land, only the conditions are much less complex. For in the case of the sea, difference in the temperature of the water may be said to constitute almost the only * Trans. of Edin. Geol. Soc., vol. ii. p. 252.

difference of climatic conditions. If there is one fact more clearly brought out than another by the recent deep-sea explorations, it is this, that nothing exercises so much influence on organic life in the ocean as the temperature of the water. In fact, so much is this the case, that warm zones were found to be almost equivalent to zones of life. It was found that even the enormous pressure at the bottom of the ocean does not exercise so much influence on life as the temperature of the water. There are few, I presume, who reflect on the subject that will not readily admit that, whether as regards the great physical changes which are taking place on the surface of our globe, or as regards the growth and distribution of plant and animal life, the ordinary climatic agents are the real agents at work, and that, compared with them, all other agencies sink into insignificance.

It will also be admitted that what holds true of the present holds equally true of the past. Climatic agents are not only now the most important and influential; they have been so during all past geological ages. They were so during the Cainozoic as much as during the present; and there is no reason for supposing they were otherwise during the remoter Mesozoic and Paleozoic epochs. They have been the principal factors concerned in that long succession of events and changes which have taken place since the time of the solidification of the earth's crust. The stratified rocks of the globe contain all the records which now remain of their action, and it is the special duty of the geologist to investigate and read those records. It will be at once admitted that in order to a proper understanding of the events embodied in these records, an acquaintance with the agencies by which they were produced is of the utmost importance. In fact, it is only by this means that we can hope to arrive at their rational explanation. A knowledge of the agents, and of the laws of their operations, is, in all the physical sciences, the means by which we arrive at a rational comprehension of the effects produced. If we have before us some complex and intricate effects which have beer