191 1.] REID— ISOSTASY AXD ^lOUXTAIX RANGES. 449 



or decrease. If by erosional transportation a large quantity of 

 material is removed from a high land and deposited in the oceans, 

 then the increase of weight under the ocean and the decrease under 

 the mountains will, as ]\Iajor Dutton explained, set up a subter- 

 ranean counter flow, which will restore the equality of material in 

 the segments. If by the exercise of tangential forces a portion of 

 the earth's crust is compressed and folded and the quantity of 

 material in the segment thus increased, the added weight will cause 

 a slow sinking of the region and material will flow out from below 

 and reduce the mass of the segment to its proper value. Indeed, 

 the folding up of the rock by tangential pressure would not elevate 

 a mountain range, but would cause the folded region to sink ; not, 

 however, necessarily below its former level. 



When we consider the origin of the mountain ranges the theory 

 of isostasy requires that all hypotheses, which call for more than 

 the normal amount of material in any segment, be excluded. The 

 folding of rock under tangential forces, and the increase of material 

 by subterranean flow are necessarily debarred. Dana noticed that 

 the great mountain ranges of the world were opposite the great 

 oceans and, in some cases, were opposite the great depths of the 

 oceans. The inference was natural that material was taken from 

 the ocean bed, increasing its depth and added to the land increasing 

 its height ; but the theory of isostasy forbids this inference. He also 

 suggested that the segments of the earth forming the oceans were 

 sinking more rapidly, as the earth cooled, than the segments forming 

 the continents and also that they were stronger ; so that they com- 

 pressed the continents, folding the rock and making mountain ranges 

 around their borders. Besides other objections to this idea, the 

 theory of isostasy excludes it on account of the increased material 

 required in the land segment. Professor Charles Davison- has sug- 

 gested^ that the oceans owe their existence to the stretching and 

 consequent thinning of the strata below them, but the theory of 

 isostasy does not permit the withdrawal of material from the ocean 



* " On the Distribution of Strain in the Earth's Crust resulting from 

 Secular Cooling, with special reference to the Growth of Continents and 

 the Formation of Mountain Chains," Phil. Trans. R. S., 1887, Vol. 178(A), 

 pp. 231-242. 



