538 Henry Woodtvard — Address to the Geologists' Association. 



bably take place under heavy pressure, even at ordinary temperature, 

 but is no doubt hastened by high temperature. Such a temperature 

 as 800° is certainly sufficient to produce not only metamorphism, but 

 aqueo-igneous pastiness, or even complete aqueo-igueous fusion. 

 With a small quantity of alkali in the included water of such sedi- 

 ments, all these changes would take place at a much lower temper- 

 ature. 



Suppose then sediments accumulating along the shores of a conti- 

 nent : the first effect is segregation, and therefore increasing density, 

 and consequent contraction and subsidence pari passu with the deposit. 

 Next, if the accumulation of sediment continue, follows aqueo-igneous 

 softening, or even melting, not only of the lower portion of the 

 sediments themselves, but of the underlying strata upon which they 

 are deposited. The subsidence probably continues during this process. 

 Finally this softening determines a line of yielding to horizontal 

 pressure, and a consequent up-swelling of the line into a chain. 

 Thus are accounted for, first the subsidence, then the subsequent up- 

 heaval, and also the metamorphism of the lower strata so universal 

 in great mountain-chains. By this view, of course, the exposure of 

 the metamorphic rocks on the surface is the result of subsequent 

 erosion. Even the granitic axis, Le Conte thinks, in most cases is 

 but the lowermost, and therefore the most changed portion of the 

 squeezed mass, exposed by subsequent erosion; although it is by no 

 means impossible that in some cases the granite may be squeezed out 

 as a pasty mass through a rupture at the top of the swelling mass 

 of strata. 



This theory, as will be observed, strongly inclines towards the 

 metamorphic origin of granite, but does not require it. For there 

 is nothing to hinder the aqueo-igneous fusion of an original granite 

 crust by the accumulations of sediments upon it, and the consequent 

 yielding of the crust along the line of accumulation. 



But a mountain-chain is not a simple mass of strata, which, becom- 

 ing to a certain extent plastic, from aqueo-igneous action, and yielding 

 to horizontal pressure, bends upwards into an arch, leaving a hollow 

 space beneath, nor is it such an arch filled and supported by an in- 

 terior liquid mass. On the contrary, if we examine carefully the 

 strvicture of any extensive mountain range, we shall find that it is 

 not only made up of masses of immensely-thick sediments, but that 

 these have been folded, crumpled, crushed, fissured, and faulted in 

 the most marvellous manner, whilst the intimate structure of the 

 rock has been entirely altered by having undergone slaty-cleavage, 

 produced by powerful pressure perpendicular to the planes of 

 cleavage, by which the whole rock-mass has been mashed together 

 and shortened in that direction, and correspondingly extended in the 

 direction of the planes of cleavage. As these planes of cleavage 

 are usually highly inclined, or even vertical, it is evident that the 

 rock-mass has been crushed together horizontally, and swelled up, 

 vertically. As a necessary consequence of the crushing together, we 

 find associated with cleavage the most complex foldings, not only of 

 the strata, but of the layers, and even of the finest lines of lamin- 



