In cliffs of sea-shore and river, in railway-cuttings, in mine-shafts and quarries, we may often see layers of the earth’s crust in the order of their original deposition. Except where the signs are present of some subsequent violent interference, this order is uniform and invariable. It is not that all the members of the series are invariably present, far from it; but in order of deposition the relations of higher and lower are never interchanged. Every one of the many different layers which have been distinguished by geologists has a distinctive group of fossils. You may, if you please, suppose that for each of these layers of the earth’s crust there was a new creation of living creatures, wonderfully like at each successive step, though wonderfully different at long intervals, as though they were the work of an artist whose ideas moved but slowly; but for such a supposition you have no authority; the conception has neither simplicity nor grandeur; it does not even accord with the facts, since, amid the general change of organic structures, we find the permanence of a few; and while the groups of two successive layers have, each of them, numerous distinctive forms, it is impossible to draw any definite boundary-line between the groups themselves, which sometimes intermingle with an inextricable interlacing on their confines. Nothing comes out more clearly to the student of the rocks, than that the world of to-day is the world of millions of years back; from one point of view ever-changing, yet ever essentially the same; from another point of view, out of the utmost regularity of alternation, never producing the same thing, or presenting the same aspect twice. We think that the stage has been essentially altered, because in the days of that immeasurable past _we_ did not strut upon it. We are unable to fathom the depth of our own insignificance, and are unwilling to believe in a march of time, compared with which the span of our own lives seems so contemptible. In the depths of the ocean the formation of chalk is said to be going on at this very day. Probably there is no time known to the geologist at which the formation of chalk has not been going on in the depths of the ocean; but its older layers have been altered by chemical and mechanical forces, by fire, by pressure, and by other means.