FORMATION OF MONOOLIXAL MOUNTAIN RANGES. 125 



form, the one is anticlinal, the other monoclinal. As to cause, the one 

 is formed by lateral squeezing and strata-folding, the other by lateral 

 stretching, fracturing, block-tilting, and unequal settling. As to place of 

 birth, the one is born of marginal sea bottoms, the other is formed in 

 the land crust, Classified by form, Ave may regard the two types as be- 

 longing to the same grade of earth features, namely, mountain ranges; 

 but classified by their generating forces, they belong to entirely different 

 groups of earth-movement. The one belongs to the second group men- 

 tioned above, the other to the third and fourth groups ; for the plateau- 

 lifting, crust-arching, and consequent tension and fracturing belong to 

 the third group or oscillatory movements, but the mountain-making 

 proper — that is, the subsequent block-tilting and unequal settling — be- 

 longs to the fourth group or isostasy, for that is wholly the result of 

 isostatic readjustment and is one of the best illustrations of this principle. 

 It shows on what comparatively small scale under favorable conditions 

 (probably unstable foundation) the principle of isostasy may act. 



It is evident, then, that it is impossible to exaggerate the distinction 

 between these two types of mountains. They belong, as we have seen, 

 to entirely different categories of interior forces, and, indeed, are not 

 both mountains in the same sense at all. It was for this reason that in 

 my paper on mountain structure* I put these latter in the category of 

 mountain ridges instead of mountain ranges — of modification, not of 

 formation. I now think it better to divide mountain ranges into two 

 types, not forgetting, however, the very great distinction between them.. 



Conclusions. 



To sum up, then, in a few words : There are two primary and perma- 

 nent kinds of crust movements, namely, (a) those which give rise to 

 those greatest inequalities of the earth surface— oceanic basins and con- 

 tinental surfaces ; and (b) those which by interior contraction determine 

 mountains of folded structure. These two are wholly determined by 

 interior forces affecting the earth as a whole, the one by unequal radial 

 contraction, the other by unequal concentric contraction — that is, con- 

 traction of the interior more than the exterior. There are also two 

 secondary kinds of movement, which modify and often mask the effects 

 of the other two and confuse our interpretation of them. These are (c) 

 those oscillatory movements, often affecting large areas, which have been 

 the commonest and most conspicuous of all movements in every geolog- 

 ical period, and are, indeed, the only ones distinctly observable and 



* Am. Jour. Sci., vol. 16, 1878, p. 95. 



