244 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



tary area. Since the material which flows horizontally from the 

 lower end of the block to maintain equilibrium is heavier than the 

 sediments which are laid down on its surface, there will be a less 

 volume of material removed from the block than is brought to it in 

 the form of sediments, and therefore the surface should apparently 

 be gradually increased in elevation. There must be a continuous 

 contraction of the material of the block in order that there can be a 

 continuous deposition of material at or very near sea level. This 

 would lead one to believe that the loss of heat continues after the 

 former mountain area has been depressed to sea level and during 

 the process of sedimentation of the area. 



There are two other causes of downwarping, but they are secondary 

 and not primary or fundamental. One is the weight of the sediments 

 which causes the original material of the block to sink lower into 

 the earth. The other is the compression of the material of the block 

 due to the weight of the sediments. 



The contraction of the original material of the column must be due 

 in greater part to the chemical or physical changes which must have 

 occurred as a result of the loss of much heat after the area had be- 

 come base-leveled. It will be seen from the above that the phenome- 

 non of downwarping of an area is rather complicated and it is im- 

 possible to estimate the influence of each of the several causes. 



CYCLES OF UPLIFT AND SUBSIDENCE. 



It seems probable that there is a cycle of erosion and deposition in 

 any particular region of the earth which causes a portion of the re- 

 gion to be at or below sea level at one time and much elevated at 

 another, and that these two conditions might occur a number of times 

 in the same general area. The area which is now high and subject to 

 erosion may later be the region within which new sediments will be 

 placed, and the region now undergoing sedimentation may, and prob- 

 ably will, be one of uplift in the future. There are numerous ex- 

 amples of mountain areas which have been below sea level several 

 times, as shown by the presence of sedimentary rocks of different 

 geological ages. This change in the elevation of the surface of the 

 block is apparently due to local causes and has no relation to the 

 supposed horizontal stresses resulting from a shrinking of the earth's 

 nucleus which, it has been believed, caused the uplift of mountain 

 systems as wrinkles of the earth's crust. 



The system outlined above, consisting of the erosion and sedimen- 

 tary cycles, seems to be a simple one, and there are reasons for feeling 

 that the causes are competent ones. At least, there is something 

 tangible to work with, while, with the old theory of the horizontal 

 thrusts due to shrinkage of the nucleus of the earth, it is difficult to 

 conceive how the forces acted. 



