256 STRUCTURE COMMON TO ALL ROCKS. 



1. By Fracture and Slipping. — It is obviously impossible that such 

 violent foldings of the strata should take place without frequent fract- 

 ure and slipping of the broken parts. These fractures and faults were 

 produced at the time of origin, or else during the growth of the range. 

 If the mountains are very old, erosion has long since cut down the 



inequalities thus 

 produced ; but if 

 the mountains 

 are recent, they 

 may still form 

 conspicuous oro- 



Fig. 228.— Layers of Clay folded by Lateral Pressure (after Favre). , -i • * , 



In Fig. 227 the lower part shows the Uintah Mountains as they are, 

 and the upper part shows the same as they would be if the eroded strata 

 were restored. In more complex mountains the fracturing and fault- 

 ing are also more complex. Fig. 228 shows the result of an actual 

 experimental crushing of variously-colored layers of clay. 



2. By Metamorphism. — We have said that mountain strata are often 

 of enormous thickness. We shall give abundant proof of this here- 

 after. But we have also seen (p. 221 et seq.) that the accumulation of 

 sediments to great thickness will produce a rise of the isogeotherms — 

 an invasion of the sediments with their included water by the interior 

 heat, and a consequent hydrothermal softening or incomplete hydro- 

 thermal fusion of the lower parts of such accumulations. Now we find 

 that mountain strata are nearly always more or less metamorphic in 

 their lower parts. Thus every great mountain-range has a metamor- 

 phic core. This is represented in the experimental figure (Fig. 219) 

 by the shading. 



3. By Subsequent Erosion. — The modifications thus far spoken of 

 were produced at the time of preparation or else in the origin and 

 growth of the mountain, and therefore belong to the category of mount- 

 ain formation. But so soon as the mountain begins to rise, it begins 

 to be sculptured by erosion ; and when we remember that, on account 

 of their great elevation and steep slopes, mountains must be the the- 

 atres of the greatest activity of erosion, it is evident that the meta- 

 morphic core will often be exposed by erosion along the crests. Thus 

 the typical structure of a great mountain-range is that of a meta- 

 morphic or granitic axis emerging along the crest and flanked on 

 each side by strata corresponding to one another. It was formerly 

 supposed that the granitic axis was pushed up from below, breaking 

 through the strata and appearing above them. But it is far more 

 probable that the so-called granitic axis is only the metamorphic core 

 formed as already explained, and exposed by subsequent erosion. Fig. 

 229 is an ideal of a mountain-range on this view. In this case the 



