VI PREFACE. 



accordantly maintains that they have been simply mineralized 

 " sandstones have been converted into quartzites," and so on, 

 using almost the same terms as Lyell did, and after the lapse 

 of nearly half a century. 



This latter-day exposition of metamorphism, the writers feel, 

 affords indubitable proof that the present work is imperatively 

 called for to meet the requirements of a large body of geological 

 students. 



Having laboured at the subject for fifteen years (aided to 

 some extent by a grant from the Government Fund), they have 

 determined the existence of two strongly differentiated groups 

 of metamorphic rocks mineralized and methylosed. They are 

 also enabled to show that the characters of the latter group 

 have been developed by the decomposing action of carbonated 

 solutions on siliceous minerals ; that essentially silacid rocks, 

 which have been permeated by solvents of the kind, have be- 

 come variously transmuted and transformed in the one case 

 having had their mineral silicates for the most part replaced 

 by mineral carbonates (calcite &c.), in the other the same 

 siliceous minerals, where simply affected by partial erosion and 

 replacement, having become shaped into a variety of residual 

 configurations that have been mistaken for organic structures, 

 as in " Eozoon Canadense." Moreover the above pheno- 

 mena enable the writers to show that both bedded and dyke- 

 shaped masses on a large scale exhibit corresponding chemical 

 changes ; so that what were once essentially siliceous rocks are 

 now ophites, hemithrenes, and the like, in which it cannot be 

 said that " there is little or no development of new material," 

 and which, in short, clearly prove that there are numerous 

 other metamorphic rocks besides those that are simply mine- 

 ralized. 



Just before the (t creature of the dawn " made its advent, 

 mineral pseudomorphism was labouring under great bewilder- 



