340 A. KEITH OUTLINES OF APPALACHIAX STRUCTrRE 



in the schists, introduce some ease of motion, although not to the same 

 extent as in sedimentary layers because of the interlocking of the min- 

 eral grains. The fact that they did yield is obvious wherever the gneisses 

 and granite were brought together in the Appalachian deformation. 



Yielding is equally obvious in the Triassic deformation where faults 

 cutting 20,000 feet of strata or more were imposed on the Triassic sedi- 

 ments by the Precambrian granites. Furthermore, it is shown in the 

 numerous synclines of Paleozoic rocks infolded in ^ the Precambrian. On 

 all of these the trend of the Precambrian was imposed, as has been al- 

 ready desci^ibed. The overwhelming power of the thrust by the Pre- 

 cambrian granites is shown by the fact that the greatest known over- 

 thrust in the Appalachians extends visibly down into the granites which 

 formed the overriding mass. Summing up, therefore, it seems clear from 

 theory, from detailed observation, and from a variety of generalizations 

 that the Precambrian granites were the rocks which transmitted the 

 pressure to the Aj^palachians : that the more massive groups of the Lower 

 Paleozoic sediments transmitted the forces upward and northwestward, 

 and that the weaker beds which constitute the Middle and Upper Paleo- 

 zoic were comparatively passive. 



The conclusions stated in the preceding i^aragraph appear to account 

 completely for the phenomena of the closely folded belt of the Appa- 

 lachians. In this belt the element of lateral shortening overshadows that 

 of vertical uplift. The latter, however, is l)y no means inconsiderable 

 but is to be measured in miles. The uplift as measured by reconstruct- 

 ing the folds is from 5 to 10 miles or more, according to the region 

 selected and to the principles followed in restoring the folds. This 

 amount is to be compared Avith the 200 miles of lateral shortening already 

 deduced. In view of this great shortening and the piling up which it 

 must have caused, it is seen that from 5 to 10 miles is a conservative 

 estimate of the heights to which the folds would have extended unless 

 eroded. 



The proportion of vertical to horizontal motion changes rapidly north- 

 westward and is reversed in the plateau region. In fact, the amount o^' 

 lateral shortening on the various great domes like the Cincinnati, Xash- 

 ville, Ozark, and Wisconsin uplifts is measured in thousands of feet. 

 The Cincinnati dome, though often called an arch, has in reality sucli a 

 slight curvature (one or two degrees from the horizontal) that no known 

 rocks could support themselves on this arch Ijy their own strength. They 

 must, therefore, in this arch and in similar ones, some gentler and some 

 more pronounced, l)e supported by pressure from below. It is equally 

 clear that horizontal pressure alone could not raise such an arch l)ecause 



