OROGRAPFIY, or OR LOGY, from Spos a mountain, and 7paysi description, and aleyes, discourse, respectively, (the science of mountains,) belongs technically to three branches of natural knowledge, physical geography, meteorology, and geology; but it is usually regarded as a part of the first, and confined to the subjects of the configuration, magnitude, and distribution of mountains, with some general reference, also, to their climate, and to their relations to organic nature.
Though the term mountain be universally understood, yet it will be found very difficult to define strictly what is meant by it. From the mole-hill in the meadow, to the gigantic Himalayas, the gradations are infinite, and no positive line can be drawn between the hill and the mountain. Moreover, the name is sometimes given to a single eleva tion or peak, as Mount rEtna, &c., and sometimes to a whole and extensive cluster of eminences, as Mount Caucasus. Isolated moun tains are rare, and when they do exist are generally, though not always, volcanic.
The general disposition of mountains is in groups or chains. The word group explains itself, but what is understood by a chain may not be so clear. When hills or mountains are so arranged as to form a line or band whose length greatly exceeds its breadth, such a dispo sition, whether the lino be straight or curved, is called a chain, and sometimes a ridge; though the latter name is more exclusively applied to the lesser chains. We are not however to understand by a chain of mountains, a single unbroken longitudinal eminence, like that farmed by the connected roofs of a row of houses. A chain of mountains, on the contrary, is very irregular, and composed of many subordinate parts. In what is regarded by some geographers as a complete chain there are three parallel or nearly parallel ridges; the centre one is usually the highest. These three ridges, though distinct, are seldom equidistant from each other, and they are frequently united. From the points of junction, and from different parts of the outer ridges, other chains strike off at various angles, and these in turn send off other ramifications, which go on dividing and diminishing in height till the last undulations are lost in the general surface of the plain.
These different branches of a chain have received various and very arbitrary names. Some divide the whole system of a chain into the primary, principal, primordial, or great chain, and secondary and tertiary chains ; but these names having now reference to the order of formation of rocks rather than to their disposition, though the latter circumstance is greatly dependent on the former, they are at present confined to geological considerations, and we therefore say such a chain or ridge is a branch of some other chain or ridge. Thus the Apen nines are a branch of the Alps. Minor ramifications, when short, are called spurs. Wherever the main chain sends off other chains, the former is, at that particular spot, higher than elsewhere, so that between two consecutive elevations there is an apparent depression : hence the summit or ridge-line of the main chain is divided into heights, which are called peaks, domes, &c., according to their shape; and depressions, which receive the general name of passes, because they are the places where the passage over the chain from opposite valleys is most easily effected. These passages or passes are termed cob in the Alps, ports in the Pyrenees, and pertuis in the Jura. They are oleo called gorges or defiles, but incorrectly, for the gorge is properly the contracted part of a valley, and the defile, a very narrow passage at the foot of the mountains or winding amongst them. The ridge-line of great chains L. also irregular in breadth as well as height ; it is sometimes very narrow and sometimes very broad, and in soma places spreads out into what is termed table-land (Langfiehl in Norway).
fable-Lands, or plateaus, are however not always of this terrace form : they are not unfrequantly sunk, if we may so say, into the broad summit of the ridge, so as to be surrounded by lofty eminences and peaks. Tableland is also sometimes ascended by gentle acclivities, without any appearance of mountains.