c, gives a tolerably correct account of the well-known countries of the world, and of the Mediterranean, Euxine, and Caspian, together with the rivers which fall into those seas, but it added little to the knowledge of the n. of Europe, or the extreme boundaries of Asia or Africa. Yet, from his time till the 14th c., when the records of the travels of the Venetian Marco Polo opened new fields of inquiry, the statements of Ptolemy were never questioned, and even during the 15th c., it was only among a few German scholars at Nürnberg that the strange accounts given of distant eastern lands by the Venetian traveler were received as trustworthy where he differed from Ptolemy. Marco Polo had, however, unfortunately made no astronomical observations, nor had he even recorded the length of the day at any place, and hence the Nürnberg geograph ers, who had no certain data for estimating the extent of the countries which he had traversed, were the means of propagating errors which led to results that were destined to influence the history of mankind; for, taking Ptolemy's tables as their basis, they had incorporated on their globes and maps the results of their own rough estimates of the length of Marco Polo's days' journeys, and they had thus represented the continent of Asia as extending across the Pacific, and having its eastern shores somewhere in the region of the Antilles. These erroneous calculations misled Christopher Columbus to the false assumption that, by sailing 120° w., he would reach the wealthy trading marts of China, and the result of this conviction was his entering upon that memorable expedition which terminated in the discovery (in 1492) of the continent of America. Although there can be no doubt that the American continent was visited in the 9th and 10th c. by Northmen, the event remained without influence on the history of discovery, and cannot therefore detract from the claims of Columbus. This momentous dis covery, which had been preceded in 1486 by the exploration of the African coast as far as the cape of Good Hope (which was doubled by Vasco da Gama in 1497), was followed by a rapid succession of discoveries; and within 30 years of the date of the first voyage of Columbus, the whole coast of America from Greenland to cape Horn had been explored, the Pacific ocean had been navigated, and the world circumnavigated by Magellan (q.v.) the coasts of eastern Africa, Arabia, Persia, and India had been visited by the Portuguese, and numerous islands in the Indian ocean discovered. The 16th c. was marked by continued attempts, successful and unsuccessful, to extend the sphere of oceanic discovery; and the desire to reach India by a shorter route than those by the cape of Good Hope or cape Horn, led to many attempts to discover a n.w. passage, which, though they signally failed in their object, had the effect of very materially enlarging our knowledge of the arctic regions. The expeditions of Willoughby and Frobisher in 1553 and 1576, of Davis (1585), Hudson (1607), and Baffin (1616), were the most important in their results towards this end. The 17th and 18th centuries gave a new turn to the study of geography, by bringing other sciences to bear upon it, which, in their turn, derived elucidation from the extension of geographical knowledge; and it is to the aid derived from history, astronomy, and the physical and natural sciences, that we owe the completeness which has characterized modern works on geography. In the 17th c., the Dutch, under Tasman and Van Diemen, made the Australasian islands known to the civilized world; and in the latter half of the 18th c., Capt. Cook extended the great oceanic explorations by the discovery of New Zealand and many of the Polynesian groups; but he failed to find the antarctic continent, which was first visited in 1840 by American, English, and French expeditions, under their respective commanders, Wilkes, Ross, and Dumont d'Urville. Polar exploration, after having been for a time in abeyance, has within late years been vigorously prosecuted by the United States and various European countries. In America, the travels of Humboldt, Lewis and Clark, Fremont, and others, have done much to make us acquainted with broad general features, but much remains to be done in regard to special districts of Central and Southern America. In Asia, numerous travelers, geographers, and naturalists have contributed to render our knowledge precise and certain in respect to a great part of the continent, whose natural characteristics have been more especially represented by the great physicist Ritter; while we owe a large debt of gratitude to the Jesuit missionaries, whose indefatigable zeal has furnished us with a rich mass of information in regard to minor details of Asiatic life and nature. In Africa, the combined influences of a deleterious climate, and a religion hostile to European advance, have hitherto retarded explorations into the interior; but notwithstanding these obstacles, much light has been thrown on the character and condition of the African continent by many of its greatest explorers-as Bruce, Park, Clapperton, Adamson, the Landers, Burton, Speke, Barth, Vogel, Livingstone, Cameron, and Stanley. In Australia, although much still remains to be done, the obscurity which had hitherto hung over the interior has been to a great extent diminished by the explora tions of Sturt, Eyre, Leichhardt, and the brothers Gregory; and still more by the highly important labors of Burke and Wills, who in 1860 crossed the Australian continent from Melbourne to Carpentaria. Although both these intrepid explorers perished miserably from starvation on their return route, their journals and the description that has been given by them and their sole-surviving companion, King, of the country through which they passed are of value; nevertheless, they overestimate the fertility of that portion of the continent of Australia, as later explorations proved. The progress which has marked recent discovery has been materially aided by the encouragement and systematic organization which have been given to plans of explora tion by the public governments of different countries, and by the efforts of the numerous geographical societies which have been formed during the present century both at home and abroad; while the constantly increasing mass of information collected by scientific explorers is rapidly diffusing correct information in regard to distant regions, and thus effectually dispelling the numerous fallacies which have hitherto obscured the science of geography. Among the numerous works of authority on the subject of geographical discovery, the following may be consulted with advantage: Hudson's Geographi Graci minores; Précis de Géographie Universelle, by Malte Brun; Manual of Geographical Science (mathematical, physical, historical, and descriptive), 1860; Latham's Germania of Tacitus; Humboldt's Hist. crit. de l'Hist. de la Géographie, Asie, Centrale, and the Cosmos; Ritter's Asien; Kloeden's Erdkunde. The recent progress of geographical discovery may be traced in Petermann's Mittheilungen, the Geographical Magazine, and the Proceed ings of the Geographical Society. See also works by Bunbury and Kiepert (1880, 1881). GEOGRAPHY, MEDICAL. The liability of particular localities to become the centers of special diseases, or groups of diseases, has been observed from the most ancient periods, as we have excellent evidence in the Hippocratic treatise. On airs, waters, and places, one of the undoubtedly genuine works of the great Greek physician, and one of those which best sustains his traditional reputation. Now-a-days, medical geography has become a most elaborate and carefully investigated branch of medical science, the details of which, though of considerable popular interest, are far too complicated and too technical to be discussed with advantage here. The reader may be referred to the articles AGUE; CLIMATE; DYSENTERY; ENDEMIC; GOITER; LEPROSY; MIASMA; PLAGUE; REMITTENT FEVER; YELLOW FEVER, for incidental illustrations of the subject. Generally speaking, the tropics are subject to diarrheal diseases, with acute affections of the liver, and severe remittent or pestilential fevers, caused by the exalted temperature acting on the soil, and producing emanations very destructive of health; the like causes in more temperate climates causing ague and diarrhoea, especially during the summer and autumn in low-lying, ill-drained localities. Temperate climates are also subject in a peculiar degree to pulmonary diseases, and to all manner of contagious fevers, the result of overcrowding and confined air. Certain diseases, again, as goiter, leprosy, and some animal parasites (see ENTOZOA), appear to have no relation to climate, but are found to affect, more or less exclusively, certain well-defined districts of country; as in the case of the Guinea-worm, the Egyptian ophthalmia, the pellagra of Lombardy, the beri-beri of Ceylon and the Malabar coast, and the elephantiasis of the Indian peninsula generally. The best works on medical geography are those of Mühry, in Germany, and Boudet, in France, which are remarkably learned and complete treatises on the whole subject. A more recent one still is that of Dr. August Hirsch of Danzig, a work of immense labour and erudition, not yet completed. On tropical diseases generally, the English works of Annesley, Twining, Morehead, and sir Ranald Martin are of confirmed reputation. GEOLOGY (from the Greek, yn earth, and λóyos, discourse), is a science of wide scope based upon a knowledge of existing nature. It involves an acquaintance with geodesy, astronomy, physics, chemistry, mineralogy, petrography, meteorology, physiography, and the many divisions of biology. Authors have differed much in their methods of presenting the subject. Some have discussed separately the facts, principles and theories. As it is often convenient to consider in close juxtaposition the phenomenon and its explanation, it is preferable to sketch the science from the standpoint of its fundamental and related branches of knowledge under the following heads: I. structural; II. physical; III. chemical; IV. mineralogical; V. geographic; VI. climatological; VII. paleontological. I. STRUCTURAL GEOLOGY.-Structural geology is concerned with a description of the kinds of materials composing the earth and their arrangement. The earth has the shape of an oblate spheroid, the flattening being apparent at the poles. The polar diameter is about twenty-six and one-half miles shorter than the equatorial, so that a point on the surface about either pole is over thirteen miles nearer the centre of the spheroid than any part of the central protuberance. This form is that which would result naturally from the rotation of a plastic material at the present rate of movement; whence it has been concluded that the earth was formerly in a fluid condition through heat. The best physicists declare that if the earth had never been more pliable than it is now the particles would arrange themselves in this attitude; hence the old argument for an early condition of igneous fluidity from the earth's oblateness is weakened. This general form is maintained by the mobility of the ocean. Should the rate of rotation be increased, the earth would become more oblate, because of the transfer of the ocean to the equator; or should the rate be diminished the figure would be more nearly spherical, because the water would work towards the poles. Almost universally geologists believe that the earth was originally fluid from heat, and that it has been gradually cooling, at first possessing a thin crust, and then one of great thickness. Eventually the globe must become solid throughout, and correspond. ingly frigid. There is a difference of opinion as to its present condition, some believing in the existence of a thin crust from thirty to one hundred miles; others that the crust may be eight hundred or two thousand miles thick; and others, that however thick its crust, the interior is solid. For our present purpose it is needful only to speak of a crust in distinction from a heated interior, and to draw conclusions as to the character of the interior from the assumed present or former igncous condition. The mean density of the earth is about 5.6. The average density of the crust is about 2.5. Hence the weight of the interior must be greater than the average, and may be as great as 16. This greater density may be due to a condensation on account of the superincumbent weight, or the presence of heavier elements, which assumed their inferior condition by gravitation when the particles were free to change their places in the liquid. A study of the several igneous rocks leads to the belief that the outer shell may be of granitic rocks having a density of 2.6, followed by diabases or diorites with a density of 2.8, and lower still by basalts and other lavas with a density as great as 3.2. It would seem as if the outer envelope represents the oldest of the igneous rocks, which cooled and solidified first of all, and that the other zones became solid in succession, so that the order of density agrees with the known development of the melted material. The lowest of the lavas agrees quite closely in composition with some of the stony meteorites. They are peridotites. It has been found by synthetic experiments that these ancient lavas assume mineral arrangements identical with those of the stony meteorites. Now the meteorites represent the materials of which worlds are made. They fall to the earth from space whenever they come within the sphere of mundane attraction. But there are the iron meteorites also, much heavier than those just described. If the stony class represent a zone in the earth at the base of the lavas, the irons may show what the material is still lower down with the density of 7 or 8. In confirmation of this hint may be cited the existence of tons of supposed meteoric iron discovered in Greenland imbedded in basalt. These are generally regarded as of terrestrial origin, though undistinguishable in chemical and crystalline characters from the known meteorites. Iron is very abundant in the cosmic bodies, while the heavier precious metals are unknown. Hence, if the analogies may indicate results, it may be suggested that the great bulk of the earth's interior consists of iron, which may be compressed sufficiently at the centre to afford the necessary density. Lastly, the presence of a ferruginous interior enables us to understand why the earth as a whole is a magnet. That part of the earth accessible to observation is called the crust. It is known satisfactorily to the depth of nearly a mile by artificial borings and mines, and to a depth of seven thousand feet by natural cuttings in the great cañons of the west. Besides volcanic ejections, which have come from a greater depth, the geologist relies upon a study of the position of the rock-layers to determine the thickness of this crust. He finds everywhere proofs of the outcroppings at the surface of rocks that have been uplifted from great depths. By the patient comparison of thicknesses of strata with their inclinations into the earth over continents of surface, he is satisfied that he can identify as much as ten miles of rock in certain localities. When he stands upon the oldest of the rocks he cannot know the nature of the material beneath him save by a study of comparative densities; but elsewhere he may be satisfied as to the nature of the rocks at depths of one, five, or ten miles, according to circumstances; viewed in the most general way-including everything earthy or stony-they are of two sorts, stratified and unstrat ified. The first are arranged in layers called strata composed of assorted pieces, frag、 ments of older ledges, and are demonstrably of sedimentary origin. In general the older rocks have been broken, pulverized, and transported by aqueous agencies, to be depos ited as mud, clay, sand, gravel or related matter, beneath accumulations of water. Hence, they are said to be of aqueous origin. The unstratified rocks lack the arrangement by layers, and are usually made up of crystals which have developed in a molten magma. Hence, this class is supposed to be of igneous origin. Stratified Rocks.-The upper part of the crust is composed of stratified rocks, and fourfifths of the surface is covered by them. They may be arenaceous, argillaceous or calcareous, deriving their specific character from their source. Though now solid, they must have been incoherent at the outset, for they are composed of fragments of other rocks, and show upon their surfaces markings made by wind, waves, and rains; pieces of shells or other organic materials may be present; and in all respects they resemble the loose sediments now accumulating. They have become consolidated chiefly through some cementing substance, like lime, iron or silica. Heat and pressure may facilitate the process under favorable conditions. Running water with varying velocity will present alternatiovs of coarse and fine strata. Pebbles of the same size and material tend to accumulate in the same stratum, and the larger pieces are washed clean. Still, a single stratum may exhibit all variations of texture over a wide area. This fact may be illustrated by supposing many cubic yards of heterogeneous earth dumped into a river. The heavier stones will remain at the place of the plunge; those that are lighter will be carried to greater and greater distances according to their smallness. What is a conglomerate at the start may be traced through various gradations of sandstone, terminat ing in the finest clay. The coarser stones descending a mountain remain at its base, while the sands and clays are spread about the plains. Sediments may also be of chem ical origin, as beds of salt dropped from the ocean, or crystalline travertine deposited from springs. They may also be of organic origin like limestone, found in the ocean full of shells. |