i8 9 3. THE GENESIS OF MOUNTAIN RANGES. 377 



To be effective, the difference of temperature would have to be great 

 — something quite impossible. 



But it is not on radial expansion or contraction that the theory of 

 the origin of mountain ranges bearing my name hinges. Reasoners 

 on such complex problems should have the conditions fully and 

 accurately before them. 



A weli thought-out theory cannot be gaily dismissed in a few 

 short formal sentences. 



The horizontal extension of aggregate groups of sediments com- 

 pared with their maximum vertical depth is enormous ; it may be 

 200 times as great. Consider what would be the result of heating a 

 slab from below measuring 200 feet at the sides and 1 foot thick in 

 the thickest part. If it were bound at the sides in the way the 

 sediments are bound on the solid crust of the earth, the effect 

 would be to throw it into domicular folds distributed in relation 

 to the differential heating and thickness of the slab at various 

 points. Even if it were not so held, the differential heating — 

 the temperature being most intense at the thickest parts— would 

 still throw it into folds through the intense stresses and strains 

 set up. This is what happens in my view by the heating of 

 the sediments which afterwards become mountain ranges. It 

 initiates the movement, liberating other forces which continue the 

 building of the chain. I cannot pretend to go into the details here, as 

 they are of considerable complexity, and must refer those who wish 

 to pursue the subject to my work and the several papers I have at 

 various times during the last seven years given to the scientific 

 world. 



I must, however, before concluding, say a word on cumulative 

 recurrent expansion, as Professor Le Conte here finds my theory 

 obscure. This seems the more strange to me, as I actually 

 commenced my work by giving concrete examples of the effect of 

 recurrent expansion due to alternate heating and cooling. The folds 

 in this way induced in sheets of lead are given as good examples and 

 illustrated by photographs. Anyone who cares to look for such folds 

 can see them in any lead gutter, lead-lined bath, or lead-lined sink 

 exposed to varying temperatures. It is one of the plumber's arts to 

 provide as far as he can for this well-known effect of frequent 

 minute expansions ; but I have shown that such effects are not con- 

 fined to so ductile a material as lead. Terra-cotta copings are 

 frequently affected in a somewhat similar manner. They go on 

 expanding for years by infinitesimal degrees, mostly in the heat of 

 summer, until I have seen what is technically called a " ramp " — a 

 portion of the coping curving down to a lower level — lifted from one 

 to two inches from its bed. Not only so, but in some cases a time 

 comes when the bending at this point becomes a fault at one of the 

 joints (normal fault), the expansion still proceeding. I have had a 

 very interesting letter and photograph from Mr. John D. Paul, F.G.S., 



