264 STRUCTURE COMMON TO ALL ROCKS. 



with the interior. At first, and for a long time, the exterior would cool 

 fastest ; but there would inevitably, sooner or later, come a time when 

 the exterior, receiving heat from abroad (sun and space), as well as 

 from within, would assume an almost constant temperature, while the 

 interior would still continue to cool and contract. Thus, therefore, 

 after a while the interior nucleus would contract faster than the ex- 

 terior shell. It would do so, partly because it would cool faster, and 

 partly because the co-efficient of contraction of a hot body is greater 

 than that of a cooler body. Now, as soon as this condition was reached, 

 the exterior shell, following down the shrinking nucleus, would be 

 thrust upon itself by a lateral or horizontal pressure which would be 

 simply irresistible. If the earth's crust were a hundred times more 

 rigid than it is (30 times as rigid as steel, 500 to 1,000 times as rigid 

 as granite — Woodward, Science, vol. xiv, p. 167 1889), it must yield. 

 Mountain-ranges are the lines along which the yielding takes place, 

 and this yielding takes place along lines of thick sediments because 

 these are lines of weakness. 



There are several serious objections which may be brought against 

 this view : 1. Calculations seem to show that the amount of crumpling 

 and folding actually found in mountains is many times greater than 

 could be produced by the contraction of the earth by cooling. But it 

 may be answered that there may be other causes of contraction besides 

 cooling. For example, loss of constituent gases and vapors from the 

 interior of the earth, through volcanic vents and fissures, has been sug- 

 gested by O. Fisher (p. 100). 



2. Again, it has been shown by Dutton that it is impossible that the 

 effects of differential contraction should be concentrated along certain 

 lines, so as to give rise to mountain-ranges without a shearing of the 

 crust upon the interior portions, which is inadmissible if the earth be 

 solid. Instead, therefore, of conspicuous mountain-ranges, the effects 

 of differential contraction would be distributed all over the surface, 

 and be wholly imperceptible. But in answer to this it may be said 

 that there is no difficulty in the way of such shearing, and therefore 

 of such concentration of effects along certain weakest lines, if there he 

 a suh-crust liquid layer, either universal or else underlying large areas 

 of surface. 



Still other objections have been raised, but these are so recent that 

 they have not yet been sufficiently sifted by discussion to deserve men- 

 tion here. The origin of mountains by lateral pressure is a fact be- 

 yond dispute. This is the most important fact for the geologist. How 

 the lateral pressure is produced is a pure physical question which must 

 be left to the physicists to settle among themselves. 



Another Type of Mountains— Monoclinal Mountains.— We have 

 thus far spoken only of one type of mountains — by far the commonest 



