24G STRUCTURAL GEOLOGY. 



the earth, by which the crust, following down the shrink- 

 ing nucleus, is thrust upon itself laterally with irresist- 

 ible force. Mountain-ranges are the lines of yielding. 



Other Associated Phenomena. If we clearly appre- 

 hend the foregoing account of the structure and origin 

 of mountains, other associated phenomena are easily un- 

 derstood : 1. The strong bendings of the strata neces- 

 sarily produce fissures, mainly parallel to the bendings 

 i.e., to the axis of the range, and to one another. 2. 

 Since these fissures break through many miles of strata, 

 it is natural that igneous matter should come up through 

 them to the surface, and therefore that volcanic and 

 especially great fissure eruptions should be associated with 

 mountain-ranges, and that where the overflows are cut 

 away, by erosion we should find dikes. Again, 3. As the 

 mashing goes on steadily, the fissures first formed would 

 be certain to slip, and thus we find great faults often as- 

 sociated with mountains. Again, 4. The formation of a 

 fissure or the subsequent slipping of a fissure could not 

 fail to produce an earth-jar ; and thus earthquakes are 

 commonest in mountain-regions. Finally, 5. Fissures 

 which did not fill at the moment of formation by igneous 

 injection would certainly fill slowly afterward by perco- 

 lating water depositing minerals, and thus, also, mineral 

 veins are commonest in mountain-regions. 



Thus we see now the truth of the proposition with 

 which we set out, viz., that mountains are the culminat- 

 ing points the theaters of greatest activity of all geo- 

 logical agencies ; of aqueous sedimentary agencies in 

 preparation for the mountain ; of igneous agencies in 

 the birth and growth, and of aqueous erosive agencies 

 in sculpturing and final destruction of the mountain. 



Mountain- Sculpture. 



In the life-history of a mountain-range, the work of 

 water in sculpturing is no less important than the work 



