1879.] 



On Ophites and related Rocks. 



217 



the authors have been induced to make investigations as to (H) 

 " The origin of the minerals characteristic of ophites, &c, especially 

 peridote." With certain exceptions the minerals referred to are 

 considered to be of secondary origin, the exceptions being those 

 remaining unaffected by secondary agencies. Serpentines, malacolite, 

 phlogopite, chlorite, enstatite, and a number of others are all considered 

 as secondary minerals. Peridote, notwithstanding that it is generally 

 considered to be an original mineral in the same sense as the hornblende, 

 feldspar, mica, &c, of granite and other plutonic rocks, is regarded by 

 the authors as a product of alteration in all its relations, and circum- 

 stances of occurrence. Its presence in granites, basalts, and lavas has 

 given rise to the belief that it is of igneous origin : nevertheless its 

 occurrence in mineralised and methylosed rocks (gneiss, and ophite of 

 the sedimentary section) is held as proving the contrary: and the 

 authors feel themselves justified in assuming that it is as much a 

 secondary product as the zeolites and pseudomorphs found in granites, 

 basalts, and lavas. Many of the crystals occurring in basalts and 

 lavas, which have been taken for peridote, are in their opinion pseudo- 

 morphs after augite and hornblende. 



Repudiating the doctrine that the Archaean rocks are the result of 

 chemical precipitations, and entertaining the strongest doubts that 

 life Las been to any extent concerned in their formation, the authors, 

 in a chapter (I) " On the origin of the Archaean crystalline limestones 

 of Canada," apply their views on hemithrenes to the present subject; 

 and they arrive at the conclusion, from various considerations, that the 

 rocks in question are methylosed products ; but which, before this 

 change took place, existed as gneisses, hornblende-schists, and other 

 mineralised silacid metamorphics. 



The question (J) " Why limestones are so rare in formations imme- 

 diately succeeding the Areheeans " is discussed in connexion with the 

 facts that calcareous organisms are rare or not present in the forma- 

 tions referred to, and that calcareous rocks are abundant in the preceding 

 systems — the Archaeans. These facts are held to be in unison with the 

 authors' conclusions stated in the last chapter, and to favour the view 

 that the Archaean limestones with their present constitution were not 

 available as materials for the production of calcareous rocks in the 

 earliest Cambrian age. 



(K.) " The genetic difference between mineralised and methylosed 

 metamorphism " is explained by assuming that water has been an 

 important factor in both cases ; but as the minerals in the first group 

 are for the most part anhydrous or dry species, it is assumed that the 

 original (? hygroscopic) water, which its members contained in their 

 condition as sediment, was sufficient for their mineralisation ; on the 

 other hand, as the minerals composing the members of the second 

 group are chiefly hydrous, it is contended that their methylosis has 



