A CENTURY OF GEOLOGY. 439 



birth; we trace their growth, their maturity, their decay, their 

 death; we even find in the folded structure of the rock, as it were, 

 the fossil bones of extinct mountains. In a word, we are able now 

 to trace the whole life history of mountains. 



Mountains, therefore, have always been a subject of deepest 

 interest both to the popular and the scientific mind — an interest 

 intensified by the splendors of mountain scenery and the perils of 

 mountain exploration. The study of mountains is therefore coeval 

 with the study of geology. As early as the beginning of the pres- 

 ent century Constant Prevost observed that most characteristic 

 structure of mountains — viz., their folded strata — and inferred 

 their formation by lateral pressure. All subsequent writers have 

 assumed lateral pressure as somehow concerned in the formation 

 of mountains. But that the whole height of mountains is due 

 wholly to this cause was not generally admitted or even imagined 

 until recently. It was universally supposed that mountains were 

 lifted by volcanic forces from beneath, that the lifted strata broke 

 along the top of the arch, and melted matter was forced through be- 

 tween the parted strata, pushing them back and folding them on 

 each side. And hence the typical form of mountain ranges is that of 

 a granite axis along the crest and folded strata on each flank. But 

 attention has lately been drawn to the fact that some mountains, 

 as, for example, the Appalachian, the Uintah, etc., consist of folded 

 strata alone, without any granite axis. In such ranges it is plain 

 that the whole height is due not to any force acting from below, 

 but to a lateral pressure crushing and folding the strata, and a 

 corresponding thickening and bulging of the same along the line 

 of crushing. Then the idea was applied to all mountain ranges. 

 So soon as the prodigious amount of erosion suffered by mountains, 

 greater often than all that is left of them, was fully appreciated, 

 it became evident that the granite axis so characteristic of moun- 

 tains was not necessarily pushed up from beneath and protruded 

 through the parted strata, but was in many cases only a sub-moun- 

 tain core of igneous matter slowly cooled into granite and exposed 

 by subsequent erosion greatest along the crest. 



Next, attention was drawn to the enormous thickness of the 

 strata involved in the folded structure of mountains. From this 

 it became evident that the places of mountains before they were 

 formed were marginal sea bottoms off the coasts of continents, and 

 receiving the whole washings of the continents. Thus the steps 

 of the process of mountain formation were (1) accumulation of 

 sediments on offshore sea bottoms until by 'pari passu subsi- 

 dence an enormous thickness was .attained. This is the prepara- 

 tion. (2) A yielding along these lines to the increasing lateral 



