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 Phœnician 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.
Ptolemy, who flourished in Alexandria in the reign of Hadrian, is the next geographer of eminence, and he shows us something of Africa; for, in his time, the Phœnicians, in their commercial expeditions, had sailed far to the south, had reached the termination of Africa, with ocean lying all around it, and had seen the sun to the north of them. This last assertion, however, Ptolemy does not credit, and he is as skeptical of the open ocean surrounding the extremity of Africa as modern geographers and explorers have been of the existence of Kane's open Arctic Sea. He believes that what the Phœnician traders took to be the broad ocean must be part of an inland sea, corresponding to the Mediterranean, with which he was so familiar. His map includes also England, Ireland, and Scotland; and his Ultima Thule is, no doubt, the Hebrides of our days.
Our present notions of the past periods of the world's history probably bear about the same relation to the truth that these ancient geographical maps bear to the modern ones. But this should not discourage us, for, after all, those maps were in the main true as far as they went; and as the ancient geographers were laying the foundation for all our modern knowledge of the present conformation of the globe, so are the geologists of the nineteenth century preparing the ground for future investigators, whose work will be as far in advance of theirs as are the delineations of Carl Ritter, the great master of physical geography in our age, in advance of the map drawn by the old Alexandrian geographer. We shall have our geological explorers and discoverers in the lands and seas of past times, as we have had in the present,—our Columbuses, our Captain Cooks, our Livingstones in geology, as we have had in geography. There are undiscovered continents and rivers and inland seas in the past world to exercise the ingenuity, courage, and perseverance of men, after they shall have solved all the problems, sounded all the depths, and scaled all the heights of the present surface of the earth.
What has been done thus far is chiefly to classify the inequalities of the earth's surface, and to detect the different causes which have produced them. Foldings of the earth's crust, low hills, extensive plains, mountain-chains and narrow valleys, broad table-lands and wide valleys, local chimneys or volcanoes, river-beds, lake-basins, inland seas,—such are some of the phenomena which, disconnected as they seem at first glance, have nevertheless been brought under certain principles, and explained according to definite physical laws.
Formerly, men looked upon the earth as a unit in time, as the result of one creative act, with all its outlines established from the beginning. It has been the work of modern science to show that its inequalities are not contemporaneous or simultaneous, but successive, including a law of growth,—that heat and cold, and the consequent expansion and contraction of its crust, have produced wrinkles and folds upon the surface, while constant oscillations, changes of level which are even now going on, have modified its conformation, and moulded its general outline through successive ages.
In thinking of the formation of the globe, we must at once free ourselves from the erroneous impression that the crust of the earth is a solid, steadfast foundation. So far from being immovable, it has been constantly heaving and falling; and if we are not impressed by its oscillations, it is because they are not so regular or so evident to our senses as the rise and fall of the sea. The disturbances of the ocean, and the periodical advance and retreat of its tides, are known to our daily experience; we have seen it tossed into great billows by storms, or placid as a lake when undisturbed. But the crust of the earth also has had its storms, to which the tempests of the sea are as nothing,—which have thrown up mountain waves twenty thousand feet high, and fixed them where they stand, perpetual memorials of the convulsions that upheaved them. Conceive an ocean wave that should roll up for twenty thousand feet, and be petrified at its greatest height: the mountains are but the gigantic waves raised on the surface of the land by the geological tempests of past times. Besides these sudden storms of the earth's surface, there have been its gradual upheavals and depressions, going on now as steadily as ever, and which may be compared to the regular action of the tides. These, also, have had their share in determining the outlines of the continents, the height of the lands, and the depth of the seas.
Leaving aside the more general phenomena, let us look now at the formation of mountains especially. I have stated in a previous article that the relative position of the stratified and unstratified rocks gives us the key to their comparative age. To explain this I must enter into some details respecting the arrangement of stratified deposits on mountain-slopes and in mountain-chains, taking merely theoretical cases, however, to illustrate phenomena which we shall meet with repeatedly in actual facts, when studying special geological formations.