MOUNTAIN ORIGIN AND STRUCTURE. 255 



to override the Cretaceous (McConnell). The manner in which this 

 was done is illustrated on a previous page (Fig. 205). Evidently, then, 

 the whole height of the mountains mentioned above is due to lateral 

 crushing alone. 



2. Slaty Cleavage. — But there is another phenomenon associated 

 with mountains which furnishes additional proof, if any be necessary, 

 viz., slaty cleavage. This is not so universal a phenomenon as folding, 

 because the materials of strata are not always suitable for developing 

 this structure ; but where it occurs its evidence is equally convincing. 

 We have already seen (p. 183) that this structure is always produced 

 by mashing together horizontally and extension vertically. We have 

 also seen that in every case of well-developed cleavage the whole rock- 

 mass has been mashed horizontally three parts into one, and swelled 

 up vertically one part into three. Now, again, considering the thick- 

 ness of mountain strata, this is sufficient to account for the highest 

 mountains in the world. It is true we often find slaty cleavage where 

 there are now no mountains. In such cases the elevation produced 

 by the mashing has been swept away by erosion. We find only the 

 bones of the extinct mountains. 



It was once supposed that mountains were pushed up from below 

 by a vertically acting force. Hence came the word upheaval as applied 

 to mountains. The word is still used ; and there is no objection to its 

 use, if it be borne in mind that, in mountains of the structure given 

 above, the force of upheaval is not vertical but lateral. 



Modifications of the Simple Ideal given above. — Thus far, in order 

 to get a clear idea of the process and the result, we have described 

 mountains in their simplest form, and as similar in process of formation 

 and result to the experiment shown in Fig. 219. But in fact the 

 final result in Nature is complicated in many ways. Some of these com- 



Fig. 227.— Uintah Mountains— Upper Part restored, showing Fault; Lower Part showing the Pres- 

 ent Condition as produced by Erosion (after Powell). 



plications are shown in the foregoing figures of actual mountains, and 

 have been anticipated in their descriptions. It is necessary now to dis- 

 cuss these more fully : 



