ORIGIN OF MO UNTAIN RANGES. 557 



water. Now it is well known from the experiments of Daubree 

 and others, that in the presence of water, even in small quanti- 

 ties, rocks become softened and even hydrothermally fused at 

 the very moderate temperature of 400 to 8oo° F. r It is certain 

 then that such thickness of sediments as we know accumulated in 

 preparation for mountain birth, must have been softened to a 

 degree proportionate to the thickness, and therefore perhaps semi- 

 fused or even fused in their lower parts along the line of thickest 

 deposit, and therefore of greatest subsequent elevation. On 

 cooling after elevation, this sub-mountain fused or semifused 

 matter would form a granitic or metamorphic core beneath the 

 highest part. The appearance of this core as an axis along the 

 crest is the result not of up-thrust but of subsequent erosion greatest 

 along this line. 



And this, in its turn, furnishes a key to the location of mountains 

 along lines of thick sediments. For not only the lower parts of 

 such sediments but also the sea-floor on which they are laid down 

 would be hydrothermally softened or even fused. Thus would 

 be determined a line of weakness, and therefore a line of yielding 

 to lateral thrust, and therefore also a line of crushing, folding, and 

 upheaval. The folding and the upswellingand the metamorphism 

 would be greatest along the line of thickest sediments and become 

 less as we pass away from that line. In extreme cases, however, 

 the firmer lateral portions might be jammed in under the softer 

 central portions, on one or both sides, and give rise to the Fan- 

 structure character of complexly folded mountains. Or again, in 

 such cases the folds might be pushed clean over and broken at 

 the bend, and then the upper limb slidden over the lower limb 

 even for miles, forming the wonderful thrust-planes of the Alps, 

 the Appalachian and the Rocky Mountains, already described. 

 Thus the phenomena under (5) is completely explained. 



But mountains are usually asymmetric, the crest being on one 

 side. This is explained as follows : Sedimentary accumulations 

 alongshore lines are thickest nears\iox& (though not at shore) and 

 thin out slowly seaward. The cilinder-lens formed by sedimen- 

 tation is not symmetric, its thickest part being near one side, and 



