The precise chemical action upon the elements composing these various geological formations at different remote periods, is no doubt difficult to ascertain. That there always has been some chemical action going on, and that it is continually going on, is certain. How and to what extent we can judge only from the experience of actual observation in the laboratory. Mr. Crale remarks: "The whole surface of the land is exposed to chemical action of the air, and of the rainwater with its dissolved carbonic acid, and in colder countries the frost. The disintegrated matter is carried down the slopes during heavy rain; and, to a greater extent than might be supposed, especially in arid districts, by the wind; it is then transported by the streams and rivers, which when rapid deepen their channels and triturate the fragments." Darwin says: "If the theory be true" (speaking of the time elapsed since the Cambrian lowest formation) "it is indisputable that before the lowest Cambrian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day; that during these vast periods, the world swarmed with living creatures. Here we encounter a formidable objection; for it seems doubtful whether the earth, in a fit state for the habitation of living creatures, has lasted long enough. Sir W. Thompson concludes that the consolidation of the crust can hardly have occurred less than 20 or more than 400 million years ago, but probably not less than 98 or more than 200 millions of years. These very wide limits show how doubtful the data are; and other elements may have hereafter to be introduced into the problem. Mr. Crale estimates that about 60 million years have elapsed since the Cambrian period, but this, judging from the small amount of organic change since the commencement of the glacial epoch, appears a very short time for the many and great mutations of life, which have certainly occurred since the Cambrian formation; and the previous 140 millions of years can hardly be considered as sufficient for the development of the varied forms of life which already existed during the Cambrian period." It seems almost impossible for an ordinary mind to grasp the magnitude of the figures, the span of life being so short. Yet some idea may be formed when we compare the age of this earth's crust formation, the hundreds of thousands of years that passed in the evolution of man, and the brief space of time that has elapsed since he has become enabled to give an account of himself. As regards the thickness of the earth's crust, Professor Ramsey has given the maximum thickness, from actual measurement in most cases, of the successive formations in different parts of Great Britain; and this is the result: