EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 243 



simple curve of elevation coming* down again to the present condition, 

 but culminating a little later and rising a little higher than we actually 

 find it did. 



The question arises as to how great an area is necessary for the 

 operation of the principle of isostasy? What extent and degree of 

 inequality of surface may be upheld by earth rigidity alone ? 



The recent transcontinental gravitation determinations by Putnam 

 and their interpretation by Gilbert ] seem to show a degree of rigidity 

 greater than previously supposed. They seem to show that while the 

 whole continental arch is certainly sustained by isostasy — that is, by 

 deficiency of density below the sea level in that part, the continental 

 area being lighter in proportion as it is higher — yet great mountain 

 ranges like the Appalachian, Colorado, and Wasatch mountains show no 

 such means of support, but are bodily upheld by earth rigidity; and 

 even great plateaus, like the Colorado plateau, 275 miles across, are 

 largely, though not entirely, sustained in the same way. 



MONOCL1NAL MOUNTAIN RANGES. 



Until recently mountain ranges were supposed to be all made in one 

 way, namely, by lateral crushing and strata-folding and bulging along 

 the line of yielding. To Gilbert is due the credit of having first drawn 

 attention to another type, conspicuously represented only in the 

 plateau and basin region, especially the latter — that is, those produced 

 by tilting and irregular settling of the crust blocks between great fis- 

 sures. The two types of mountains are completely 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, fracturing, block-tilting, and unequal set- 

 tling. 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, mountain ranges; but classified by their generating forces, 

 they belong to entirely different groups of earth movement. The one 

 belongs to the second group mentioned above, the other to the third 

 and fourth groups; for the plateau-lifting, crust -arching, and conse- 

 quent 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 com- 

 paratively 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 



1 Gilbert, Phil. Soc. Washington, vol. 13, 1895, page 31 ; Gilbert, Jour. Geology, vol. 

 3, 1895, page 331; O. Fisher, Nature, vol, 52, 1895, page 433. 



