540 



silicate is generated, and held in solution in subterranean waters ; but, 

 owing to the probability of the solution being formed under pressure 

 and at an elevated temperature, the case cannot be considered as having 

 any relation to the presumed precipitation of serpentine. 



Fourth. Glauconite is essentially a hydrous silicate of protoxide of 

 iron and potash, with variable proportions of alumina and carbonate of 

 lime — a composition that consigns this partly chemical and partly 

 mechanical product to the same category as the preceding non-analogical 

 cases. A similar fate assuredly awaits the long-known infilling sub- 

 stance (silica, ferruginous clay, or silicate of iron — with or without 

 lime) of fossilized tbraminifers.* The casts of the Amphistegina, &c, 

 so often referred to by Dr. Carpenter, are the result — some of mecha- 

 nical, and others of chemical infiltration ; so that in either case, their 

 origin is no more than that of the commonest fossils. Poraminiferal 

 shells in Cretaceous rocks have often been mechanically filled with 

 chalk-mud. 



These are all the evidences that have been adduced to justify the 

 assumption that serpentine, pyroxene, loganite, &c, have been " directly 

 deposited as chemical precipitates from the seas in which Eozoon was 

 growing, or had only recently perished ;" and that these silicates "pene- 

 trated its chambers, pores, and canals, precisely as carbonate of lime 

 might have done."f Is it not significant that a complete collapse has 



* Mantell and Henry Deane in " Philosophical Transactions," 1846, p. 466 ; Ehren- 

 berg in 14 Berlin Monatsbericht," Feb., 1855 ; Bailey in " Proceedings of Boston Society 

 Nat. Hist.," vol. v., p. 364, 1856 ; &c. 



f " Quarterly Journal of Geological Society," vol. xxi., p. 70 ; and " Canadian 

 Naturalist," December, 1866, p. 125. 



Entertaining this " novel theory," Dr. Sterry Hunt consistently ascribes the forma- 

 tion of ophitic rocks to the direct deposition of their mineral substances, — the calcite to 

 the precipitation of carbonate of lime, and the serpentine to the precipitation of silicate 

 of magnesia. As regards the latter mineral, this theory is altogether different from that 

 maintained by Bischof, Rose, Breithaupt, and others, who regard it as being invariably 

 a pseudomorphic product — a view to which we have fully committed ourselves (see 

 <l Quarterly Journal of Geological Society," vol. xxii., p. 216). Most ophites appear to 

 have resulted, through regional chemical alteration, from hornblendic and augitic 

 rocks ; and, notwithstanding Sterry Hunt's arguments and evidences, which we have 

 shown to be untenable, it so happens that he himself has adduced facts strongly su- 

 staining the view to which he is opposed. " Large masses of white granular pyroxene 

 are frequent in beds of limestone in the Canadian Laurentians, generally associated 

 with serpentine, which often incrusts it ; and small nuclei of this pyroxene frequently 

 form the centre of concretionary masses of serpentine:" the latter "may vary from a few 

 inches to a foot or more in diameter" (" Geology of Canada," 1866, p. 205, 207). 

 It is also mentioned that " crystals of considerable size — some imperfectly defined, and 

 an inch in diameter — of serpentine, occur imbedded in calcite, in North Burgess" 

 {Op. cit, p. 204): these "crystals" may be considered as nothing but pseudomorphs. 

 His loganite, a serpentinous mineral, is considered by Dana to be an " altered horn- 

 blende" (" System of Mineralogy," 5th ed., p. 221, &c). For our part, we are strongly 

 disposed to believe that most of the minerals in the Laurentian limestones are due — some 

 directly, and others indirectly — to the pseudomorphism of doleritic, dioritic, and other 

 rocks. 



