Grolpgy. —During the azoic and pakeozoic periods of the earth's geological history, the district now occupied by the A. was a level plain. These mountains date their origin from a period subsequent to the carboniferous epoch. The coal measures are the newest upturned beds associated with the Appalachian range; and as the stratified rocks. with few exceptions, are laid down ,horizontally, these strata must owe their inclined position to the dislocating agency which elevated the mountains; they, consequently, supply a date anterior to its activity. At the base of the A., on their eastern side, there are a series of red sandstone beds, unconformable to the upturned strata, and occupying the valleys in their original horizontality, thus evidently unaffected by the disrupting agency which must have been active prior to their deposition. These beds have been referred by geologists to different ages. That they are old red sandstone, as conjectured by Machin; and others, is now universally denied. Hitchcock's supposition that they were permian, is also considered as referring them to too remote a geological age. W. B. Rogers considered them first as members of the triassic period; but has since, from evidence adduced from the contained organic remains, shown reason for relating them to the beginning of the jurassic period. We thus obtain two grand limiting dates—the carboniferous and Jurassic periods—within which the A. must have been formed. There I are grounds for being even more specific, and referring the period of the dislocating agency to that immediately subsequent to the carboniferous, represented in the stratified rocks.of other districts by the permian series; for the older upturned rock had not only been ruptured and plicated, but also denuded into the various shapes they now present, before the horizontal rocks were deposited.
The late prof. H. D. Rogers, after years of persevering and devoted study, enun ciated a theory of mountain formation based on his examination of the A., which not only explains their structure, but admits of a more or less complete application to the mountain-systems of the world. The many proposed theories of mountain elevation are based upon assumptions which, unfortunately, are not true; but that is an unimpor tant matter to the majority of our speculating geologists, and one never seen by the inventors of the theories, who allow themselves to be led captive by a poetic imagina tion, instead of building their inductions on field observations. Thus, to suppose that mountains are elevated by a wedge-like intrusion of melted matter, is to give to a fluid functions incompatible with its dynamic properties. So also the supposition that the igneous rocks were intruded as solid wedges, separating and lifting the crust, is opposed to the fact that no apparent abrasion, but generally the closest adhesion, exists at the line of contact of the igneous and stratified rocks. Equally fatal objections can be adduced against the other theories. Prof. Rogers observing that the A. were formed of a series of enormous waves, and comparing this appearance with other elevated districts, espe cially in Belgium and Britain, enunciated a theory of their structure, of which the follow ing is a condensed view: Disturbed strata have a wave-like arrangement, their dip being in curved, and never in straight planes; and in extensive areas the varying angles of dip exhibit one or more wide regular curves. These undulations are in the form of long parallel waves, their parallelism being in the line of the general trend of the part of the mountain system to which they belong. When different grades of magnitude, as regards length, height, and
amplitude, occur, the waves of the same grade are parallel, while the different grades are not necessarily so. The waves assume three different forms, which are characterized as—symmetrical flexures, equally steep on the two slopes; normal flexures, having an excess of incurvation on the one side compared with the other; and folded flexures, or ,those with a doubling under of their more incurved slopes, and among which the steepest slopes are generally directed to the same quarters. These three forms, representing dif ferent gradations in the flexure, are regular in their succession in disturbed regions, the order being the same as in the diagram—tnat is, when we start from the most disturbed side, we o from the folded waves"to the normal ones, and from these to the symmetri cal; and in the same order, the waves, as they recede from the folded side, become pro gressively wider apart and flatter. Resting on these facts, prof. Rogers advanced his view of the structure of elevated regions in the following words: "The wave-like struc ture of undulated belts of the earth's crust, is attributed to an actual pulsation in the fluid matter beneath the crust, propagated in the manner of great waves of translation from enormous ruptures occasioned by the tension of elastic matter. The forms of the waves, the close plication of the strata, and the permanent tracing of the flexures, are ascribed to the combination of an undulating and a tangential movement, accompanied by an injection of igneous veins and dikes into the rents occasioned by the bendings. This oscillation of the crust, producing an actual floating forward of the rocky part, has been, it is conceived, of the nature of that pulsation which attends all great earthquakes at the present day." This theory having originated as an explanation of the phenomena of the A., is easy of application to these mountains. They are composed of a series of parallel waves, having a general direction similar to the coast-line of the Atlantic ocean. The line of maximum disturbance is on their eastern limits; consequently, the folded flexures, with the invnr.i"" their steep sides, are chiefly confined to the great Appalachian valley, slopes s. of it. The flexures of this type impart a prevailing s.e. dip crop; their number, and the excessive difficulty of detecting and con g. them, frustrates every attempt at mapping them individually. The ,kcond type, which curve more rapidly on the one side than on the other, the forces that disturbed the crust were neither excessively intense nor is the characteristic form everywhere between the great Appalachian tllegliany mountains. It 'distinguishes not only those larger waves which 11—containing strata c. of the Susquehanna into special' basins—but the • minor undulations which throw the coal measures of these basins into groups of lesser saddles and troughs. Undulations of the first or symmetrical type occur beyond the Allegheny mountains, where two groups of them may be distinguished: the one sub dividing the bituminous coal-field, with its five very broad wares, into six successive basins; the other, composed of four equidistant and very straight undulations, traversing parts of Cambria, Indiana, Somerset, and Fayette counties.