112 WALKS AND TALKS. 



and their order of arrangement are exactly like what we found 

 in descending the Adirondacks. 



The Laurentian, therefore, has the same constitution as 

 the Adiroudacks. It is an elongated elevation, however, in- 

 stead of a cluster of peaks. It presents an anticlinal structure. 

 The oldest or bottom rocks are in the center, and rise to the 

 highest altitude. The rocks geologically higher in position 

 attain to successively lower and lower altitudes ; they dip down 

 on each side of the granitic axis, like a series of planks lean- 

 ing against a stone wall. 



This is the general plan of a mountain of upheaval. We 

 find the same in the Blue Ridge, in the ranges of the Rocky 

 Mountains and many others of the great ranges of the country. 

 Mountains, therefore, exist as long folds of the earth's crust; 

 and very generally, where one fold exists, two or three others 

 exist parallel with this, as in the Appalachians. 



It will be borne in mind always, that every orographic or 

 mountain feature has undergone a great amount of alteration. 

 The summits of the mountains have been much lowered. The 

 strata enwrapping their flanks have been cut back ; they stretch 

 to less distances than formerly toward the summit. In many 

 cases, indeed, we have room to conjecture that they extended 

 originally over the summit, and have been worn away in the 

 course of ages, uncovering the granitic nucleus only in later 

 geological time. 



In some cases, the erosion of parallel and contiguous mount- 

 ain crests has proceeded so far that the summits are lower than 

 the valleys between them. The valleys then become the mount- 

 ains, and in place of the original mountains are valleys of 

 erosion. So we sometimes find a synclinal structure in mount- 

 ains, and an anticlinal structure under valleys. Sometimes we 

 find a river cutting through a mountain from side to side and 

 from top to bottom. The Green river in Wyoming, affords n 

 striking example. It cuts through the 15,000 feet of the altitude 

 of the Unita Mountains. It seems quite evident that the river 

 was here before the mountain. The mountain rose gradually 

 under the river, and as it rose, the river sawed its gap to the 



