364 A. KEITH OUTLINES OF APPALACHIAN STKUCTUKE 



the moving of the crust on the interior of the earth. This is, of course, 

 proportional to the weight and thus to the thickness of the crust involved. 

 Xeither Taylor nor Wegener gives any idea of the thickness which he 

 regards as likely, but appears to regard it as a superficial matter. It is, 

 nevertheless, a matter of prime, and even of destructive, importance, in 

 view of the minute force relied on to cause the creep. Thus, the moving 

 crust required by Taylor, thickening to the south from a minimum of 

 three miles in Greenland, would acquire a prohibitive thickness long 

 before it reached southern Asia. AYegener makes no statements as to 

 the thickness of the crust, but his diagrams convey the idea of a -crust of 

 light rocks floating with comparative freedom in a sea of heavy rocks. 

 This arrangement, to his mind, appears to require no particular friction, 

 but such a conception will hardly be accepted by scientists in general. 

 Lambert states, moreover, that the horizontal pull available from density 

 differences is only one-millionth of an atmosphere — a strikingly small 

 force. In fact, Daly, in an attempt to render this hypothesis workable 

 by getting rid of this enormous friction, has postulated a layer of glass 

 between the crust and the deeper earth which would afford a minimum 

 of friction. This is, however, only a postulate and can hardly be sup- 

 ported by evidence. Thus, the ease of motion which a lateral creep of 

 the continents would require is an obstacle which is likely to remain a 

 permanent one. It is also a real difficulty for all theories. 



It will be evident from the foregoing summary that no theory of 

 mountain-building is free from serious objections. In some cases the 

 objections are far more Aveighty than the arguments in favor, while in 

 others there is a body of probability. Such a situation is only to be ex- 

 pected from the nature of the process, which took place in the depths of 

 the earth and whose results have in large measure remained there. How- 

 ever Avell supported a theory may be by mathematics, it can not prevail 

 unless it satisfies the conditions laid down by the visible rocks, and it is 

 for that reason that the tests by geologic facts which are knowii and can 

 be reaffirmed have been made the prime consideration in this discussion. 

 The theory which is to last must explain all the facts, but first those of 

 geolog}', for the problem of mountain -building would have been unknown 

 except through the rocks. We may well wish for more facts, and we are 

 gradually getting them. Accordingly, the theory proposed in the next 

 section has been derived directly from the geologic facts. It appears to 

 satisfy the phenomena of the Appalachians, not only the usual but the 

 exceptional ones, and it may solve the difficulties in other mountain 

 ranges. 



