Theories of the EarWs Physical Evolution. 375 



expansion of the underlying magma, involves at once the conclusion, 

 that the magma must ultimately reach a degree of density much 

 less than that of the overlying rocks, which break up into prisms or 

 folds, and sink or recoil away from the axis of the rising colloid 

 mass. The objection might be raised, that the materials of mountain 

 cores are now as dense as those of the adjoining strata, and where 

 they are cold and crystalline, they are so. But according to this 

 argument, the case is quite different at an inconsiderable depth. 

 Beneath the mountain crests, the colloid mass preserves its greater 

 volume near to the surface ; beneath the adjoining strata it has been 

 protruded laterally away and extravasated ; the quantities of matter 

 in the two columns being equal. Of this an indication is found in 

 the very rapid rate of increase of temperature with depth which is 

 found in mines sunk in the granitoid cores and flank, s of such 

 mountains. 



Volcanos are the extreme cases of an extreme class. The alliga- 

 tion of a large quantity of water in the lava renders it of compara- 

 tively small specific gravity, and it needs for the expelling force, 

 even from a lofty summit, only the weight of the strata which cover 

 the liquid reservoir. Volcanos occur where hydrothermal lavas 

 exist, and are displaced by the subsidence of sediments. Hence 

 their situation in mid ocean, or on its shores, or even upon the 

 shores of inland lakes, where deposits are going on abundantly. 

 And the author illustrates this view by several American examples. 

 Perhaps as good a sample as an}^ is the Uintah range. Disregarding 

 the enormous Cretaceous deposits, the fresh-water Tertiaries turned 

 up on the flanks of these mountains are 10,000 feet thick. That 

 these beds subsided by their gross weight as rapidly as they grew, 

 admits of no shadow of doubt. The mountain cores against which 

 they recline are rocks, which give every indication of having been 

 extravasated upwards. What became of the matter displaced by 

 the sinking strata, and whence came the displaced matter which 

 slopes down to their upturned edges, and how can the conclusion be 

 avoided that they are one and the same ? If it be objected that the 

 greater altitude of the mountains should overbalance the weight of 

 the lower strata, the reply is : The mountains are there, and at their 

 feet are the sunken strata. Grant that the density of the mountains 

 is less, and the argument becomes a demonstration. Conformably 

 to this view, the position of volcanos around the outskirts of 

 maximum sediments, which are usually shore deposits, becomes a 

 normal one. The liquidity of lavas renders their extravasation 

 easy at the points of greatest relief, where rents are most liable to 

 occur ; and their lower density is a sufficient reason for the altitude 

 at which they are poured out. 



Captain Dutton does not claim for the above argument that it is 

 a solution of the general problem of the evolution of the earth's 

 physical features, but intends it as an attempt to indicate a highly 

 iaiportant consideration hitherto neglected. And he considers that 

 it rests upon facts of geological structure, that are held to admit of 

 no other possible interpretation. 



