Pre- Cambrian Hocks, etc. 277 



Palaeozoic and Mesozoic sediments getting more and more into 

 disfavour every year, and if his diagenic notions as to the origin of 

 serpentine are not yet generally accepted, be can point to several 

 men of eminence who hold these views with more or less modifi- 

 cation. 



Subjoined is an abstract of the author's summary of the chief 

 points regarding serpentine and ophiolitic rocks as set forth in the 

 paper before us : — 



1. History of opinion. 



2. To show how, from the hjTDothesis of their eruptive origin, came the appli- 

 cation of that of metasomatosis, and also to set forth the hypothesis of the 

 aqueous origin of serpentine, explaining how silicates of magnesia may, on chemical 

 grounds, be looked for at any geological horizon. 



3. To indicate the various horizons at which serpentines are found in the 

 ArchaBan rocks of North America, all being regarded as indigenous stratified rocks. 



4. The occurrence of serpentine amongst the gj-psiferous rocks of the Silurian 

 series at Syracuse, N.Y. 



0. Having noticed some points regarding the nomenclature of serpentine and 

 related rocks, and Bonney's account of the serpentines of Cornwall, and parts of 

 Italy, the author considers the serpentine-bearing rocks of the Alps, in which he 

 shows four great groups in ascending order, viz. (a) ihe older gneiss, {b) the 

 pietre-verdi or greenstone series, (c) the newer gneisses and mica schists, and {d) the 

 still younger lustrous schists. Of these b, c and d contain interstratified serpentines ; 

 the youngest of these groups includes the marbles of Carrara. 



6. This youngest group not Mesozoic ; relations of these crystalline schists to the 

 fossiliferous rocks of the mainland of Italy and the islands ; their Pre- Cambrian age. 

 The ophiolites, etc., which have been referred to the Tertiaries, are but exposed 

 portions of these Pre-Cambrian rocks. 



7. The crystalline rocks of the Simplon and the St. Gothard, and those of Saxony 

 and Bavaria are considered, and are compared with the younger gneisses of North 

 America. 



8. The serpentine mass of Monteferrato regarded as Archtean. 



9. The genesis of serpentines. 



10. The geognostical history of olivine is discussed, and the essentially Neptimian 

 origin of many olivine rocks maintained. 



11. Geognostical relations of serpentine: the appearances of intrusion to be ex- 

 plained by subsequent movements of the strata in which the serpentines are included. 



The geological history of serpentines, as above set forth by Di-. 

 Hunt, may therefore be divided into two principal branches, one of 

 which deals with theories as to the origin of the rock, and is mainly 

 chemical ; the other, relating to its geognosy, is intimately mixed up 

 with the study of the Archgean rocks, in which, for the most part, 

 the serpentines are said to be interstratified. 



The Genesis of Serpentine. — Out of some half-dozen theories, ac- 

 counting for this most unaccountable rock, mentioned by Dr. Hunt, 

 we may select three principal ones for consideration. 



1. The most improbable is the doctrine of unlimited substitution, 

 whereby an integral conversion of ordinary types of felspathic rocks 

 into serpentine takes place througb the complete elimination of the 

 alumina, alkalies, and lime, and the replacement of these bases by 

 magnesia. Both Sterry Hunt and his antagonist, Professor Bonney, 

 are strongly opposed, as is well known, to this hypothesis, which 

 the former characterizes as nearly obsolete. Not but what rock 

 substitution has certainly taken place on a considerable scale, whilst 

 pseudomorphs of serpentine after pyroxene and even more aluminous 



