252 



STRUCTURE COMMON TO ALL ROCKS. 



3 ^ 



lateral pressure — this lies still in the field of discussion. We shall dis- 

 cuss it briefly in its proper place. 



Mountain Structure. 

 A mountain-range, then, may be regarded as a mass of enormously 

 thick strata crushed together laterally and swelled up along the line of 

 crushing. We have said that this mode of origin is revealed in its 

 structure. We can best make this plain by an experiment. Suppose, 

 then, we place, one atop another, several layers of any plastic substance, 

 such as wax, so as to make together a prismatic mass, as represented in 

 section in Fig. 219, A, and the whole resting on a smooth oiled slab of 



glass or steel, so that 



-5 — there shall be no 



friction or adhesion. 

 Suppose, further, 

 that very gentle heat 

 be applied beneath 

 along the middle 

 line, so as to soften 

 slightly this part. 

 Of course, such soft- 

 ening would be 

 greatest at the bottom, and become less and less upward ; also greatest 

 along the middle line, and become less and less outward. This is rep- 

 resented in the figure by the shading and in nature by the metamor- 

 phic softening, of which we will speak later. Suppose, now, we place a 

 board on each side of the prismatic mass, and press gradually together. 

 All the layers will be thickened and folded, and the whole mass swelled 

 up along the central line into something like Fig. 219, B. We have in 

 miniature both the structure and the mode of formation of a mountain- 

 range. In a similar way, but on a larger scale, all great mountain-ranges 

 seem to have been formed. 



Fig. 219. 



Fig. 220.— Ideal Section across the Uintah Mountains (after Powell). 



