DISTRIBUTION OF TIIE MATERIALS OF THE EARTH. 01 bases on which the strata are placed; while, in other cases, under differences of constitution and in their periods of formation, they repose on the stratified rocks. In neither case therefore, are they limited to the higher grounds; as the positive altitude of rocks, or their relation to the' surface of the earth, does not necessarily correspond with their geological altitude, or with that relative elevation which they possess in the series. For the same reason they do not invariably form mountains, although commonly occurring in mountainous countries. Lastlv, they form but a small apparent part of the visible surface; whatever reasons we may have for believing that they occupy a great extent in the regions inaccessible to our sight or operations. The great bulk of the accessible surface of the solid earth, is composed of stratified rocks, which, under different modes of distribution, form, not only the low plains, but the elevated mountains: being brought into view by their irregularities of position, and by that destruction which has so often laid them bare, and has generated the lower materials which, in other parts, conceal them from immediate examination. To their variety of position is principally owing that inequality in the surface of the earth, by which it swells into hills or rises into mountains; although these forms have been, in a greater or less degree, influenced and modified by the actions which are daily operating on the surface; transferring the ma terials of the elevated grounds to the plains and valleys below, and burying many of them beneath the depths of the sea. Geology has distinguished these strata, according to their relative seras of formation, into primary and secondary: but each of these, and the latter in particular, involve subsidiary distinctions