326 Revieivs-Capt. C. E. Button— 



assigned to the required data, some one or more of them — the time, or 

 the difference of conductivity, or that of the mean temperature of 

 different portions of the crust — will be involved to an extent far 

 greater than physicists or geologists will be disposed to grant. 



2. The subject of plications is one to which the contractional 

 hypothesis is supposed to be especially applicable. The astonishing 

 attitudes, the extreme disturbance and manifestations of energy, 

 which they frequently display with endless diversity, fill the mind 

 of the geologist with wonder and awe. Out of the vast array of 

 facts the following generalizations clearly appear; — 1. They are 

 aggregated in great belts, where the folds lie near and approximately 

 parallel to each other, with a high degree of flexure ; while the 

 neighbouring regions are comparatively but little so disturbed. 

 2, Plications are the invariable concomitant of mountain forms. 

 Sometimes the folds themselves form the mountains and ridges. 

 Sometimes they lie upon the flanks of ranges, which are composed 

 of granite cores, which seem to have risen up out of the dej^ths, and 

 pushed the strata aside to find exit. There is, however, almost an 

 exception to this generalization in the country drained by the 

 Colorado river. Here, and also in the Great Basin, the rocks are 

 cut by great faults, and uplifted thousands of feet in tables, with 

 strata nearly horizontal, or tilted so that one side of a range shows 

 the surface, and the other the faulted edges of the strata. The con- 

 verse pi'oposition, that plicated regions are mountainous, is not 

 always true. 3. Plications occur in regions of maximum sedimenta- 

 tion — that is, where large bodies of sediment have been accumulated 

 rapidly. The Palseozoic strata of Great Britain, and the Appa- 

 lachians, are ten or fifteen times as thick as those of the Mississippi 

 Valley and of Central Eussia. The former are plicated ; the latter 

 not so. The Alps, Jura, Urals, Himalayas, Andes, and Eocky 

 Mountains, are all plicated, and are regions of maximum sedimenta- 

 tion. The reverse proposition is also true, that regions of minimum 

 sedimentation are comparatively undisturbed. 4. The epochs at 

 which the plications commenced were those during, or immediatel}^ 

 following, the deposition of maximum sediments. It has frequently 

 happened that a region has received deposits through a long series 

 of epochs, and in such cases a general unconformity shows the pli- 

 cations to have been continuous also. This generalization is im- 

 portant ; for it suggests that the relation is something more than a 

 mere coincidence. 5. In some plicated regions there may be ob- 

 served a marked coincidence, or general parallelism between the 

 directions of the axes of flexure and the axes of maximum sedimen- 

 tation. This is the case with the Appalachians, the Alps, and the 

 Jura. In the Eocky Mountains the thicker strata do not occur in 

 elongated narrow belts having a uniform trend ; but are broad and 

 irregularly defined. But there is also less regularity in the plications 

 and forms of disturbance. 



Now the contractional theory assumes plicated regions to occur 

 along lines of weakness in the crust. But does it suggest any 

 reason why those places where the strata lie thickest should become 



