STRUCTURES COMMON TO ALL ROCKS. 39 



the Sierra, the Coast ranges, and many others. So the 

 Alps, the Himalayas, and the Andes consist also of sev- 

 eral parallel ranges. 



A mountain-range is one of these great components, 

 formed at one time by one earth-effort (monogenetic), 

 though the effort may have continued through a great 

 period of time. The Colorado, the Uintah, the Wah- 

 satch, the Sierra, and the Coast ranges are good examples. 

 The Blue Eidge and the Alleghany ranges are also good 

 examples. 



A mountain-ridge is a subdivision, again, of a range, 

 produced usually by erosion, although sometimes also by 

 foldings of strata. Mountain-peaks are serrations of the 

 crest of a range or a ridge, either by erosion or by volcanic 

 ejections. 



Mountain-systems are separated by great interior conti- 

 nental basins ; mountain-ranges by great valleys ; moun- 

 tain ridges and peaks by narrow valleys or gorges. 



Such is the simplest view of the form of mountains ; 

 but sometimes a mountain-range seems to be composed of 

 an inextricable tangle of ridges running in all directions. 



Now, it is evident that any scientific discussion of the 

 origin and structure of mountains must be essentially that 

 of the origin and structure of ranges; for, on the one 

 hand, a mountain-system is a mere adding of range to 

 range, and, on the other, ridges and peaks are the result 

 of subsequent sculpturing by rain and rivers. It is of 

 ranges, therefore, that we shall mainly speak. 



The surface of the earth has now become cool and its 

 mean temperature fixed, and is, therefore, no longer con- 

 tracting but the interior is certainly still extremely hot, 

 and still cooling and contracting. The effect of such 

 interior contraction is to thrust the exterior crust upon 

 itself horizontally with irresistible force, crushing it to- 

 gether with many complex foldings of strata, and caus- 

 ing it to bulge up in long wrinkles. Such lines of bulging 



