STRATIGRAPHICAL AND OROGENIC RELATIONS. 19 



under a deep cover of overlying strata and therefore at temperatures which 

 would in a large measure diminish their rigidity. This does not seem a 

 satisfactory explanation, for the reason tliat while the amount of strata which 

 has been removed from these eastern mountains has probably been larger 

 than is the case with the western ranges, the erosive process, at many points 

 in the west, has gone far enough to reveal the bases of the anticlines and 

 synclines, even as it has done along the Atlantic coast. It has been noted 

 that the eastern basins are generally, except perhaps that of the Dan River, 

 much more intersected by dikes and stocks than are those of the west, where 

 there are but few such intrusions known, perhaps in all not over a dozen 

 between the Catskills and Ala- 

 bama, as against the thousands 

 which may be traced in the moun- 

 tain-built area of the Atlantic coast. 



XL Seems l'eaSOnaOie TO aSSUme Fig. i Diagram of assumed conditions of compressive strain i 



,i , + t i • 1 .• rocks in a basin of accumulation. A A, massive crystalline 



tliat Tile extensive piaSTlC move- rocks. BB, the rocks of the basins. The arrows indicate the 



mentS Of the rocks in the East AID- <> ir6cti ° n '° f ^compressive grains, the spaces between their 



1 heads indicate the measure ol tin' .yielding at the several points. 



palachian district are related to the 



igneous action which has occurred in this field, and that the two groups of 

 facts show that the modes of action of the mountain-building forces in the 

 two districts were in some ways very different. I venture to suggest that 

 the difference was partly due to the conditions of the superficial rocks in the 

 two fields. In the west the surface of the country from northern New York 

 to Alabama was, at the time of the elevation of the West Appalachians, again 

 occupied by relatively unbroken strata which lay on the surface of the upper 

 Paleozoic rocks. The stresses of compression which assailed this wide field 

 when the conditions of resistance were uniform affected all parts of it in 

 an approximately equal degree. In a limited way, in northern Alabama, 

 Georgia, and'Tennessee, where the conditions were diversified, the stresses, as 

 shown by Hayes, were in some cases locally accumulated and so discharged 

 as to bring about extensive overthrusting; but in general the folding was 

 approximately equal for each unit of the section which was stressed. There 

 was, in a word, no transfer of thrust through great beams of massive and 

 therefore unyielding rock to fields where the stress could take effect on the 

 easily folded strata On the Atlantic slope, however, the conditions led to 

 the local intensification of the stress phenomena. Between the deep basins, 



