JS^O. 5.] HUNT — PRE-CAMBRIAN ROCKS. 259 



sustained by the analogies offered in cases of local hydro-thermal 

 action on sediments, and by the resemblances which reeomposed 

 materials frequently offer to their parent crystalline rocks. It 

 is here maintained that the great formations of stratiform crys- 

 talline feldspathic, hornblendic and micaceous rocks, which, in 

 various parts of the world, have been alternately described as 

 plutonic masses, and as metamorphosed paleozoic, mesozoic or 

 cenozoic strata are, in all cases, neptunean rocks, pre-Cambrian 

 or pre-Silurian in age, and that we know of no uncrystalline 

 sediments which are their stratigraphical equivalents. 



We have then before us two schools, the one maintaining the 

 secondary origin of a great, and, by them, undefined' portion of 

 the crystalline stratiform rocks, while assigning to certain older 

 (pre-Cambrian) crystalline rocks (of which they admit the ex- 

 istence), either a neptunean or a plutonic origin. The other, or 

 plutonist school, while asserting the plutonic derivation of the 

 greater part of the crystalline formations, accepts, to some extent 

 also, the notion of secondary and neptunean metamorphic schists. 

 It is believed that the above concise statements cover the ground 

 held by the hitherto prevailing neptunean and plutonist schools, 

 neither of which, it is maintained, expresses correctly the present 

 state of our knowledge. In opposition to both of these are the 

 views taught for the last twenty years by the writer, and now ac- 

 cepted by many geologists, which may be thus defined : — 



1st. All gneisses, petrosilexes, hornblendic and micaceous 

 schists,* olivines, serpentines, and in short, all silicated crystalline 

 stratified rocks, are of neptunean origin, and are not primarily 

 due to metamorphosis or to metasomatosis either of ordinary 

 aqueous sediments or of volcanic materials. 



2d. The chemical and mechanical conditions under which these 

 rocks were deposited and crystallized, whether in shallow waters, 

 or in abyssal depths (where pressure greatly influences chemical 



* It is a question how far the origin of such crystalline aluminous 

 silicates as muscovite, margarodite, damourite, pyrophyllite, kyanite, 

 fibrolite and andalusite is to be sought in a process of diagenesis in 

 ordinary aqueous sediments holding the ruins of more or less com. 

 pletely decayed feldspars. Other aluminous rock-forming silicates, 

 such as chlorites and magnesian micas, are however connected, 

 through aluminiferous amphiboles, with the non-aluminous magne- 

 sian silicates, and to all of these various magnesian minerals a very 

 • different origin must be ascribed. 



