THEORIES AT PEESENT IX VOGUE. 9 



zone. The different portions of the sediments -would be more or less affected 

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

 imity to the hj'pothetical zone. If they came within the zone, their fusion 

 or solution would give rise to Lavas and volcanic eruptions. Some authors 

 hold that every form of eruptive rock comes from sedimentary materials 

 -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- 

 tino-uishable from those of tlie 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. Tliis 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 were deposited in pre-Cambriau 

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

 by aqueo-igneous solution or fusion. These crystalline rocks and their 

 derived eruptive forms are then divided according to their lithological 

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

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

 of dikes and lava-Hows. 



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 points, 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 Avhich they had been subjected. 

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

 pect to find every degi-ee 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. 



