and the Origin of Mountain Ranges. 213 



brings further heated masses to increase the temperature of the 

 sediment and crust below it. I have shown experimentally that 

 every variation of heat which takes place goes for mountain 

 building, an increase producing a further compression of the folds 

 and a decrease compressive extension or normal faulting. But I 

 feel that time is short, and I must conclude without oppressing you 

 with further details. 



Besume. 



We have thus seen that changes caused by a fluctuating and 

 irregular distribution of temperature in the crust through unsym- 

 metrical cooling is a much more potent factor in producing the 

 rugosities of the earth's surface than symmetrical secular contraction. 

 There are other phenomena that it is necessary to take into con- 

 sideration in any complete theory of mountain formation, but I shall 

 merely note them here. The fact of the existence of piles of 

 sediment 10 miles deep, or twice the depth of the deepest parts of 

 the existing oceans, shows that the crust has sunk in such a locus to 

 an abnormal extent. That this is due partly to the weight of the 

 sediments is admitted by most geologists. 



With a shell of contraction of the thickness I have shown, it is easy 

 to see that any irregular distribution of weight must aifect the crust 

 by bending it, and this means also a disturbance of heated matter 

 below. All these disturbances lead to heating and cooling of local 

 areas, and this I have shown is the most potent cause of orographic 

 changes. These lateral expansions take the form of ridging up ; 

 matter is actually moved towards the axis of expansion so that the 

 ridgings up become permanent features until removed by denudation. 

 The result of all the investigations of underground temperature show 

 that the rate of augmentation downwards is very variable in different 

 localities. We also know from the existence of volcanic tracts that 

 the heated matter of the globe is there nearer the surface, and I 

 have little doubt that the increase of heat downwards in certain 

 areas is much greater than 1° F. per 50 feet.^ No doubt the cooling 

 of the earth takes place on the whole as inferred by Lord Kelvin ; 

 but sedimentation as one of the conditioning agents has not 

 received the attention its importance demands. The earth, as I 

 have shown, is covered with sedimentary matter in thickness to 

 be measured by miles. Sediments many miles thick have been 

 laid down, upheaved, crumpled and folded, denuded and again 

 covered up by other sediments, the ruins of the old, often with 

 volcanic additions from below. 



The crust of the earth is in a constant state of change, none the 

 less effective because slow as measured by our conceptions of time. 

 The unloading of one portion of the earth's crust and the loading of 

 another, the checking of the outflow of heat by sediments in one 

 area and its acceleration in another by denudation, the movement of 



1 Prof. Joseph Prestwich, in an exhaustive discussion of underground temperature 

 ohservations, suggests that 45 feet per degree is nearer to the true normal. — -Proc. 

 Eoyal See. 1886. 



