COXTRACTIOX SUBOCEAXIC SPREAD 345 



to satisfy such motion was very great, if not too great. Since, however, 

 as has been shown on previons pages, single thrusts of 20 miles throw 

 are known in the Appalachians, and thrusts are probable which on a 

 single plane exceed the total contraction previously assigned to the 

 Appalachians, it is evident that the theory has not been fairly tested in 

 a quantitative way. When the theory is confronted with a probable 

 shortening of 200 miles in the Appalachians, a situation results which is 

 certainly serious for the theory. Geometry points out to us very simply 

 that one mile of shrinkage toward the center of the earth wdll produce 

 3.14 miles of horizontal shortening, if all the shortening is concentrated 

 into one belt. Thus the shortening visible in the Appalachians requires 

 that the crust shall have shrunk radially 63 miles. No calculations by 

 physicists permit a shrinkage during the Paleozoic that even faintly 

 resembles this, and studies by geologists have found reasonable perma- 

 nence in the earth, instead of the vast changes that would ensue from 

 such a shrinkage. 



While this process may have operated to a slight extent, it clearly is 

 not the principal factor in Appalachian deformation. It should be borne 

 in mind that this test of the theory applies to the deformation at the end 

 of the Paleozoic. The principal deformation preceding that was in the 

 Precambrian, so that only that shrinkage which accumulated during the 

 Paleozoic was concerned in the Appalachian revolution. He would be a 

 bold man indeed who would assert that during Paleozoic time the earth 

 has shrunk 63 miles radially, or even a small fraction of that amount. 

 On the contrar}^ the dynamic history of the earth during the Paleozoic, 

 as interpreted by a great number of geologists and from many angles, is 

 one of moderate movement, with tvro exceptions, and only slight modifi- 

 cation of the landmasses. The general shape of the continents and the 

 seas remain roughly the same. In meeting the test of Appalachian 

 structure, therefore, the theory of radial shrinkage as a major cause of 

 folding is seen to fail completel3\ 



SUBOCEAXIC SPREAD 



The theory of suboceanic spread makes the fundamental requirement 

 that beneath the oceans the density is greater than elsewhere, a relation 

 that appears to be supported by a good basis of fact. The test of the 

 validity of the theory is the arrangement of its results with reference to 

 the oceans and the particular nature of the results from place to place. 

 The general arrangement is in accord with the theory, in that great 

 deformations have been and are marginal to the oceans. The detailed 

 facts, however, are not so in accord. The Appalachians, for instance, 



