244 EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



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, 1 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 forget- 

 ting, however, the very great distinction between them. 



CONCLUSIONS. 



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

 manent kinds of crust movements, namely: (a) Those which give rise 

 to those greatest inequalities of the earth's surface — oceanic basins and 

 continental surfaces; and (b) 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 unequal 

 radial contraction, the other by unequal concentric contraction; that 

 is, contraction 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 geological period, and are, indeed, the only ones distinctly observ- 

 able and measureable at the present time, but for which no adequate 

 cause has been assigned and no tenable theory proposed; and (d) iso- 

 static movements or gravitative readjustments, by transfer of load from 

 place to place, by erosion and sedimentation, or else loading and unload- 

 ing 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 effects of («) continent and ocean basin- 

 making, this last (d), isostasy, has concealed the effects and obscured 

 the interpretation of all the others, but especially of (b 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. 



1 Am. Jour. Sci., vol. 16, 1878, page 95. 



