DISTRIBUTION OF THE MATERIALS OF THE EARTH. 61 



bases on which the strata arc 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 invariahly form mountains, although commonly 

 occurring in mountainous countries. Lastly, they 

 form hut a small apparent part of the visihle surface ; 

 whatever reasons we may have for helieving 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 aeras of formation, 

 into primary and secondary: but each of these, and 

 the latter in particular, involve subsidiary distinctions 



