and the Origin of Mountain Ranges. 211 



and islands, and tbey occupy in addition unknown regions under the 

 seas and oceans. The Cretaceous and Tertiary rocks cover, or have 

 covered, at least half the continental area of North America, extending 

 through 60 degrees of latitude and being a thousand miles wide in 

 the United States. 



It is in this area the Rockies and related mountain ranges have 

 been thrown up, involving Cretaceous and Tertiary rocks in their 

 folds. 



For the purpose of illustrating the effects of sedimentation on the 

 earth's outer envelope as compared with that due to secular con- 

 traction we will assume an area 2000 miles long, 1000 miles wide, 

 and 10 miles thick along a central longitudinal trough, but gradually 

 thinning in lenticular but irregular fashion towards the edges. Let 

 us further assume that 4 miles represents the average thickness of this 

 group of sedimentary strata. This would augment the temperature of 

 the bed-rock on which the sediment rests by 400° at the rate of 1° F. 

 per 50 feet. This accession of heat would represent a linear expan- 

 sion in one direction of 22,000 feet and in the other of 11,000. So 

 that we have for one group of rocks, over what is only a local area 

 of the globe or -aV of the total land surface, a linear expansion of 

 4 miles in one direction and over 2 miles in the other to contrast 

 with the 15 miles surface contraction of the whole globe, including; 

 land areas and ocean, since the beginning of the Cambrian. 



The shortening of this area, if the shrinkage of the globe were 

 equally divided over its surface, would only amount to -f of a mile in 

 one direction and IJ miles in the other, through all the time that 

 has elapsed since the commencement of the Cambrian. But for a 

 true comparison with our sedimentary example, taking into account 

 geological time, we may divide this b}^ not less than 5, reducing the 

 shortening of the area on the hypothesis of contraction to respectively 

 ^ and ^ of a mile. But when we come to contrast with this the 

 effects of cubical or voluminal expansion instead of linear, it will be 

 seen that the thermal expansion of the rocks due to sedimentation is 

 still more in excess of any possible accumulation of volume due to 

 the shell of compression as possibly affected by such contraction. 



The increase of temperature is not, as I have before explained, 

 limited to the sediments ; it affects in turn the whole cooling shell 

 below and raises its temperature in a corresponding degree. 1 think 

 it will be quite fair to take this as equal to raising the temperature 

 of the outer envelope 400° to a depth of 20 miles. These are only 

 intended to be illustrative figures, applying to a small section of 

 the earth in space and a small section in time. If we cube up 

 our hypothetical block when expanded, and compare it with its 

 volume before expansion, we shall find that we have in round figures 

 241,000 cubic miles for mountain building and other orographic 

 changes, as against 26,000 cubic miles produced by the approach of 

 the shell of compression over a similar area to a similar extent ; 

 while in the second case of |- and ^ of a mile the disparity is so 

 enormously increased as to show its absolute incapacity to explain 

 any orographic features whatever. 



