29. And thus the uniformitarian is led to consider the facts which geology brings to light, as indicating a slow and almost imperceptible, but, upon the whole, constant series of changes, not only in the position of the earth's materials, but in its animal and vegetable population. Land becomes sea and sea becomes land; the beds of oceans are elevated into mountain regions, carrying with them the remains of their inhabitants; sheets of lava pour from volcanic vents and overwhelm the seats of life; and these, again, become fields of vegetation; or, it may be, descend to the depths of the sea, and are overgrown with groves of coral; lakes are filled with sediment, imbedding the remains of land animals, and form the museums of future zoologists; the deltas of mighty rivers become the centres of continents, and are excavated as coal-fields by men in remote ages. And yet all this time, so slow is the change, that man is unaware such changes are going on. He knows that the mountains of Scandinavia are rising out of the Baltic at the rate of a few feet in a century; he knows that the fertile slope of Etna has been growing for thousands of years by the addition of lava streams and parasitic volcanos; he knows that the delta of the Mississippi accumulates hundreds of miles of vegetable matter every generation; he knows that the shores of Europe are yielding to the sea; but all these appear to him minute items, not worth summing; infinitesimal quantities, which he cannot integrate. And so, in truth, they are, for him. His ephemeral existence does not allow him to form a just conception, in any ordinary state of mind, of the effects of this constant agency of change, working through countless thousands of years. But Time, inexhausted and unremitting, sums the series, integrates the formula of change; and thus passes, with sure though noiseless progress, from one geological epoch to another.