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 Phoenicians, 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 Phoenician 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.