_Theories of Geological Climate._—We have already seen that the geological agents are chiefly the ordinary climatic agents. Consequently, the main principles of geology must be the laws of the climatic agents, or some logical deductions from them. It therefore follows that, in order to a purely scientific geology, the grand problem must be one of geological climate. It is through geological climate that we can hope to arrive ultimately at principles which will afford a rational explanation of the multifarious facts which have been accumulating during the past century. The facts of geology are as essential to the establishment of the principles, as the facts of heat, light, and electricity are essential to the establishment of the principles of these sciences. A theory of geological climate devised without reference to the facts would be about as worthless as a theory of heat or of electricity devised without reference to the facts of these sciences. It has all along been an admitted opinion among geologists that the climatic condition of our globe has not, during past ages, been uniformly the same as at present. For a long time it was supposed that during the Cambrian, Silurian, and other early geological periods, the climate of our globe was much hotter than now, and that ever since it has been gradually becoming cooler. And this high temperature of Palæozoic ages was generally referred to the influence of the earth’s internal heat. It has, however, been proved by Sir William Thomson[2] that the general climate of our globe could not have been sensibly affected by internal heat at any time more than ten thousand years after the commencement of the solidification of the surface. This physicist has proved that the present influence of internal heat on the temperature amounts to about only 1/75th of a degree. Not only is the theory of internal heat now generally abandoned, but it is admitted that we have no good geological evidence that climate was much hotter during Palæozoic ages than now; and much less, that it has been becoming _uniformly_ colder. The great discovery of the glacial epoch, and more lately that of a mild and temperate condition of climate extending during the Miocene and other periods to North Greenland, have introduced a complete revolution of ideas in reference to geological climate. Those discoveries showed that our globe has not only undergone changes of climate, but changes of the most extraordinary character. They showed that at one time not only an arctic condition of climate prevailed in our island, but that the greater part of the temperate region down to comparatively low latitudes was buried under ice, while at other periods Greenland and the Arctic regions, probably up to the North Pole, were not only free from ice, but were covered with a rich and luxuriant vegetation.