ON THE ORIGIN OP MOUNTAIN EANGBS. 



157 



faco originates in a fault, this is tlio only waj in wliicli a 

 trno escarpment can arise. 



Tims to expansion of tlio nndorlayors do Mr. Reado's 

 mountains owe their l)lftli. i5at when the energies finally 

 dio out, conti-action ocoars on a largo scale, and tlms are 

 produced those normal faults which intersect or run parallel 

 with mountain axes. It might be thouglit that subsequent 

 contraction would neutralize the effects of previous expan- 

 sion, and that the mountain range would sink down once 

 more into the bosom of the continent. But Mr. Reade's ex- 

 por'imonts on sheets of load show " that that metal, oven in 

 small pieces, cannot pull itself back to its original shape on 

 cooling. Mow much less must this bo the case witli rock ! " 

 "Every fall of temperature produces aproporticmato vortical 

 subsidence of the surface over the district aifectcd ; but as 

 the materials laterally ridged up in mountain ranges by 

 expansion cannot be drawn back again during contraction, 

 thoT'o remains a permanent total of uplift in the range 

 with every rise of tempei'ature, that can only be removed 

 by atmospheric denudation." 



I have now given, I thiidc, a fair though nocessai'ily con- 

 densed account of Mr. Reade's hypothesis. I pass theroforo 

 from the expository to the critical pai't of this paper. 



W(! have soon tlia.t the author says that increase of volume, 

 as the final result of expansion (1), has hitherto remained 

 unnoticed, and (2), must be the most potent of all in its 

 geological r(iKnHis. l!nt (I), this cubical expansion with 

 consequent deformation was clearly described fourteen years 

 ago by Mr. Fisher, in a paper in tho Geological Magazine for 

 Juno, 187;5, to which Mr. Reado refers in his text. Mr. 

 Fisher says : " Kow, fixing our thoughts upon a cube of one 

 foot of rock, it certainly seems probable that, under great 

 horizontal pressure, it would undergo the small amount of 



