'I'llKDRIKS AT I'lJKSKN'l' IX VrKIl'K. 



zone. The different portions of the ^cdimentH would he more or lens alleetod 

 and metamorphosed, according- to their chemical constitution, and their prox- 

 imity to tlie hypothetical zone. If they came within the zone, tlieir fusion 

 or solution would give rise to lavas and volcanic eruptions, hjome authors 

 hold that every form of eruptive rotdc comes from sedimentary nuiterials 

 which have been thus acted upon ; while others maintain that, although the 

 true lavas and intrusive rocks may have been derived from non-sedimen- 

 tary material, yet the sedimentary rocks take upon themselves forms undis- 

 tinguishable from those of the volcanic rocks. Other modifications of this 

 theory are delegating the source of the eruptive rocks to re-fused portions 

 of the original solidified crust of the earth, which has been fused again on 

 account of relief from pressure by denudation. This last view has been 

 founded, so far as present evidence shows, on a misconception of the apparent 

 general action of matter in passing from a liquid to a solid (not cold) state; 

 therefore this should be abandoned, and fusion by increase of pressure either 

 through the earth's contraction or by the deposition of sediments substi- 

 tuted. Another theoretical view is simply a remodelling of the old Werne- 

 rian hypothesis, and its application to the crystalline rocks. According to 

 this view we are taught that all these rocks Avere deposited in pre-Cambrian 

 time, and that all eruptive rocks have been derived from these chemical ones 

 by aqueo-igneous solution or fusion. These crj^stalline rocks and their 

 derived eruptive forms are then divided according to their lithological 

 characters into distinct geological ages, and their nge is said to be recogniz- 

 able whether the rocks themselves be seen in their original form or in that 

 of dikes and lava-llows. 



If the above views are correct, we should expect to find in some form- 

 ations rocks which had suffered every degree of alteration, the same rock 

 passing from an unmetamorphosed condition into a highly metamorphosed or 

 even eruptive one, with every gradation between. At certain point\ when 

 denudation has succeeded a former epoch of accumulation, the more or less 

 deeply buried sediments would again appear upon the surface, showing 

 greater or less evidence of the conditions to which they had been subjected. 

 By carefully selecting the localities to be studied, we naturally should ex- 

 pect to find every degree of change in the rocks, and various transitions by 

 direct passage from rocks unmistakably sedimentary into those that are 

 truly eruptive, in their present position, — from those rocks whose original 

 fragmental structure is undoubted, to those that have been in a plastic. 



