A chapter on mountains will not be an inappropriate introduction to that part of the world's history on which we are now entering, when the great inequalities of the earth's surface began to make their appearance; and before giving any special account of the geological succession in Europe, I will say something of the formation of mountains in general, and of the men whose investigations first gave us the clue to the intricacies of their structure. It has been the work of the nineteenth century to decipher the history of the mountains, to smooth out these wrinkles in the crust of the earth, to show that there was a time when they did not exist, to decide at least comparatively upon their age, and to detect the forces which have produced them. But while I speak of the reconstructive labors of the geologist with so much confidence, because to my mind they reveal an intelligible coherence in the whole physical history of the world, yet I am well aware that there are many and wide gaps in our knowledge to be filled up. All the attempts to represent the appearance of the earth in past periods by means of geological maps are, of course, but approximations of the truth, and will compare with those of future times, when the phenomena are better understood, much as our present geographical maps, the result of repeated surveys and of the most accurate measurements, compare with those of the ancients. Homer's world was a flat expanse, surrounded by ocean, of which Greece was the centre. Asia Minor, the Ægean Islands, Egypt, part of Italy and Sicily, the Mediterranean and the Black Sea filled out and completed his map. Hecatæus, the Greek historian and geographer, who lived more than five hundred years before Christ, had not enlarged it much. He was, to be sure, a voyager on the Mediterranean, and had an idea of the extent of Italy. Acquaintance with Phoenician merchants also had enlarged his knowledge of the world; Sardinia, Corsica, and Spain were known to him; he was familiar with the Black and Red Seas; and though an indentation on his map in the neighborhood of the Caspian would seem to indicate that he was aware of the existence of this sea also, it is not otherwise marked. Herodotus makes a considerable advance beyond his predecessors: the Caspian Sea has a place on his map; Asia is sketched out, including the Persian Gulf with the large rivers pouring into it; and the course of the Ganges is traced, though he makes it flow east and empty into the Pacific, instead of turning southward and emptying into the Indian Ocean. Eratosthenes, two centuries before Christ, is the first geographer who makes some attempt to determine the trend of the land and water, presenting a suggestion that the earth is broader in one direction than in the other. In his map, he adds also the geographical results derived from the expeditions of Alexander the Great.