240 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



is brought into the base of the block under the mountain area, in 

 order to restore the isostatic equilibrium, the length of the block will 

 gradually diminish and the area will cease to be different in its ele- 

 vation from the surrounding region. 



MOUNTAIN SYSTEMS NOT CAUSED BY SIDE THRUSTS. 



As there is to-day an isostatic balancing of the blocks of the earth's 

 crust under various classes of topography (that is, different degrees 

 of elevation), and as it is reasonable to assume that this balance has 

 obtained in past geological periods, we have to conclude that moun- 



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/ 



/ 



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V 



V 



Fig. 5. — Distortion of crust incident to mountain formation by horizontal thrust. 



This diagram indicates graphically the distortion that would take place in the earth's 

 crust if the uplift of a mountain area were due entirely to horizontal thrusts of a 

 regional nature. The central oval in a full line represents the mountain area — about 

 200 miles wide and 1,000 miles long. The full straight lines represent imaginary lines 

 approximately 200 miles apart, placed on the earth's surface before movement began. 

 The dotted lines indicate positions of full lines after uplift. The material of the 

 earth's crust which is supposed by some to carry thrusts competent to raise a mountain 

 mass must be so yielding as to undergo the distortions indicated. Instead of the earth's 

 crust being contracted in a horizontal direction from the thrusts it would actually be 

 expanded. 



tain systems are not due to horizontal movements of the crust result- 

 ing from forces acting in a horizontal direction from great distances 

 away from the affected regions. Should there be such horizontal 

 movements, we would be forced to believe that extra loads were placed 

 on blocks which were previously in isostatic equilibrium. We do not 

 have blocks that depart materially from the isostatic equilibrium, 

 and therefore we must eliminate horizontal forces of a regional nature 

 as the cause of the uplift of a mountain system. 



If we eliminate the horizontal thrust theory of the formation of 

 mountain systems, we must adopt the vertical movement theory. The 



