330 



SCIENCE. 



[N. S. Vol. V. No. 113. 



duced by tilting and irregular settling of 

 the crust blocks between great fissures. 

 The two tj^pes of mountains are com- 

 pletely contrasted in all respects. As to 

 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, fractur- 

 ing, 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, we may 

 regard the two types as belonging to the 

 same grade of earth features, namely, moun- 

 tain ranges ; bat classified by their gener- 

 ating forces, they belong to entirely different 

 groups of earth movement. The one be- 

 longs to the second group mentioned 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 — belongs 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 maj^ 

 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 moun- 

 tain 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 for- 

 getting, however, the very great distinction 

 between them. 



Am. Jour. Sci., Vol. 16, 1878, p. 95. 



Conelusions. 

 To sum up, then, in a few words : 

 There are two primary and permanent 

 kinds of crust movements, namely: (a) 

 those which give rise to those greatest ine- 

 qualities of the earth's surface — oceanic 

 basins and continental surfaces ; and (6) 

 those which by interior contraction deter- 

 mine mountains of folded structure. These 

 two are wholly determined by interior forces 

 affecting the earth as a whole, the one by un- 

 equal radial contraction, the other by un- 

 equal concentric contraction — that is, con- 

 traction of the interior more than the ex- 

 terior. 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 geological period, and are, indeed, 

 the only ones distinctly observable and 

 measurable at the present time, but for 

 which no adequate cause has been assigned 

 and no tenable theory proposed ; and (cZ) 

 isostatic movements or gravitative readjust- 

 ments, by transfer of load from place to 

 place, by erosion and sedimentation, or else 

 loading and unloading by ice accumulation 

 and removal, and also by readjustment of 

 great crust blocks. If the previous one (c) 

 or oscillatory movements have masked and 

 so obscured the efiects of (a) continent and 

 ocean basin-making, this last (rf), isostasy, 

 has concealed the effects and obscured the 

 interpretation of all the others, but espe- 

 ciallj' of {h and c) mountain-making forces 

 and the forces of oscillatory movements. 

 In fact, in the minds of some recent writers 

 it has well-nigh monopolized the whole 

 field of crust movements. We shall not 

 make secure progress until we keep these 

 several kinds of movements and their 

 causes distinct in our minds. 



Joseph Le Conte. 



