Horace B. Woodward— On the Metamorphism of Strata. 405 



Professor Ramsay^ has shown that throughout the period occupied 

 by the deposition of the New Eed Marls and Lias, the general 

 tendency of the area was one of depression, and that after the 

 deposition of the Lias the general tendency was for a lengthened 

 time one of elevation, the successive Oolitic deposits accumulating in 

 a diminishing area. 



The beds in question may, therefore, have been upheaved into 

 their present relative position, and altered in Oolitic times. 



This was attended by much faulting, as is apparent when a 

 survey of the ground is made. Indeed, the neighbourhood of these 

 cherty beds is much affected by faults, which are probably more 

 numerous than can be ascertained with certainty. 



Metamorphic Agents. — Elevation we know to be due to igneous 

 agency, and, therefore it is to the presence of igneous matter that 

 we attribute the alteration of these Secondary strata. While, 

 however, we attribxite the metamorphism to igneous eruption, it is 

 only as a promoting cause, for we are led from the absence of any 

 traces of igneous rock at the surface to infer that the heat which 

 produced the change was applied through the agency of water. 



Dana^ has pointed out how sub-marine beds are saturated with 

 water, and how, when an eruption should occur, and the molten 

 rock is forced up through fissures, the interspersed or permeating 

 waters, as well as those superincumbent, are heated, and they con- 

 vey the heat far into the rock, and produce metamorphism. In this 

 way he explains how certain soft clays have been changed tO' a bluish 

 chert. 



These heated waters would, as Da,na remarks, contain much silica 

 in solution which they have taken from the siliceous materials at 

 hand. So that any addition of this substance to the metamorphosed 

 rock is satisfactorily accounted for. 



The patchy nature of the metamorphism, which is not due to 

 denudation, although the beds have suffered considerable erosion, 

 may be partly accounted for by local igneous protrusion, partly by 

 the facts that metamorphism is greatly influenced by mineral cha- 

 racter, so that sets of beds which are altered and not altered in turn 

 have been observed.^ 



In introducing such theoretical notions to explain the origin of 

 the metamorphism of these Secondary beds, in the area to which 

 they have been applied, little else but negative evidence can be ad- 

 duced in their support ; the only apology that can be made for such 

 temerity is that in so doing, we have endeavoured to show how 

 much still remains to be done in elucidating the geological history 

 even of a district so much studied as the Mendip Hills, 



^ Mem. Geol. Survey, vol, i., p. 297. 



2 American Journal of Science, vol. xlv. 



^ A great many valuable facts relating to metamorphism have been brought 

 together in a paper by Dr. J. J. Bigsby, Edin. New Phil. Joxirn., new series, April, 

 1863. 



