HOW DEFORMING FORCE WAS TRANSMITTED 341 



the beds are not competent to stand the strain. Furthermore, no ar- 

 rangement of pressures can be determined which would focus on a single 

 area from all sides so as to raise a symmetrical dome like the Cincinnati 

 and other domes. 



Still less is this possible when several domes are considered — for in- 

 stance, the Cincinnati, Xashville, and Ozark domes. How is the pressure 

 northward to the Cincinnati dome and southward to the ISTashville dome 

 to be derived from the narrow saddle between them? Similarly, how 

 is an eastward pressure against the Cincinnati dome and a southwestward 

 pressure against the Ozarks to be derived from the gentle basin of the 

 Illinois coal measures? It seems necessary to conclude, therefore, that 

 a considerable amount of vertical pressure was applied to raise these 

 domes which could not raise themselves by horizontal pressure, yet these 

 domes and their connecting basins fall so plainly into a system with the 

 Appalachian major structures that we must conclude a community of 

 origin for the respective forces. Where free, however, to assign different 

 proportions to the two major elements of the forces, lateral and vertical, 

 the only requirement in connecting the two kinds of deformation is a 

 suitable means for transmitting the pressure and a suitable cause for 

 localizing it under the respective domes. The medium is attained in the 

 granitic rocks of the Precambrian. Three of the great domes, the Wis- 

 consin, Ozark, and Adirondack domes, exhibit the Precambrian at their 

 cores. The Cincinnati and N'ashville domes do not, but there are no 

 other differences, and this one appears to be due to their lesser height 

 and thicker sediments, so that erosion has not yet exposed the granite. 

 The cause of the- vertical pressure will be discussed on later pages in 

 connection with that of the horizontal pressure. 



The sum total of the Precambrian strength is so vastly greater than 

 that of the Paleozoic beds that the Paleozoics inevitably had to yield 

 when the pressure was applied. The effectiveness of the Precambrian 

 rocks, moreover, was enormously amplified by their great mass which far 

 exceeds that of any rigid unit in the Paleozoic section. 



Theories as to Cause of deformixg Forces 



early theories 



The beginning of structural geology was made when it was recognized 

 that beds now high above the sea were originally deposited on the sea- 

 floor. A corrollary of that concept was that each stratum when deposited 

 approached a horizontal position, just as do the beds now being formed. 

 The departure of the beds from such a position was seen to require the 



