MOUNTAIN FORMATION BY VERTICAL VS. HORIZONTAL FORCES 281 



the results of the isostatic investigations, then we are justified in con- 

 cluding that probably the earth's crust has been in the same condition of 

 isostatic equilibrium throughout geological ages. We can not conceive 

 of the physical conditions of the earth's material being very different 

 since the earliest geological record. 



If we have had the isostatic condition throughout geological times, 

 then we are forced to the conclusion that mountain masses have never 

 been extra loads on the earth's crust, and therefore that they are not due 

 to regional horizontal forces sending the earth's material into great ele- 

 vations. If we can not have mountains formed by horizontally acting 

 forces, then we are compelled to accept the theory that they are due to 

 local vertical forces and movements. 



As mountain systems have always been formed in areas which were 

 previously occupied by heavy sediments, there must have been a change 

 in the density of material during the process of mountain uplift. Do we 

 not have here the key to the situation ? We know that the density in the 

 earth's crust is a function of the elevation of the surface, and we know 

 that this surface has changed greatly in elevation from one geological 

 period to another, and that, in the process of changing a density to a 

 smaller one, movement must take place. We have here, I believe, the 

 cause of the uplift, which is the lengthening of the column under the 

 sedimentary material, as the material expands and decreases its density. 



Distortion of the sedimentary strata of the earth's crust has generally 

 been held to be largely due to regional horizontal forces. It would appear 

 that much of this distortion could be caused by vertical movements. 

 Sedimentary matter is generally deposited in a very irregular way. As 

 material is deposited in different parts of a region at different times and 

 at different rates, we should expect a great amount of distortion of the 

 sedimentary strata and also of the material at the base of the sediments 

 during the period of sedimentation. The greatest distortion of the ma- 

 terial would probably be near the edges of the sedimentary area where 

 resistance to settlement would be greatest. 



When sedimentation of a region has ceased, then there is probably a 

 period of quiescence and later of uplift, During the uplift the region 

 will undoubtedly rise in different places at different times and rates. As 

 a result of this irregular upward movement, we should expect a great 

 amount of distortion of the recent sedimentary material and also of the 

 material on which the sediments had been laid. During this process of 

 uplift it is probable that at some places there would be developed forces 

 which would be inclined to the vertical, and in some cases they might be 

 even horizontal. 



