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of our Montalban rocks : the argillites were deposited as such, 

 and have suffered little alteration ; the mica slates, especially 

 the finer-grained varieties, have been derived from slaty rocks 

 which differed from the existing argillite chiefly in containing 

 a larger proportion of alkalies, and being in consequence more 

 susceptible of metamorphism ; the coarse-grained mica slate, the 

 quartzite, and the evenly bedded, laminated gneisses may 

 probably be referred back to uncrystalline deposits analogous 

 to sandstone ; while the coarse, schistose gneisses, which are 

 frequently no more crystalline than the finer-grained varieties, 

 have possibly resulted, in some instances at least, from the 

 extreme metamorphism of conglomerates. We should thus 

 find, that the cycle of sedimentation so constantly recurring in 

 the deposits of later ages is exemplified, and that, too, on a 

 grand scale, among these ancient crystallines. This conclusion 

 accords perfectly with the conditions that, we have reason to 

 believe, presided over the deposition of sediments in the Mont- 

 alban seas ; for certainly there is nothing in our knowledge of 

 the past history of the earth to warrant the supposition that the 

 process of sedimentation was materially different during Mont- 

 alban time from what it is now. For long ages, viz., since 

 the beginning of the Laurentian era, the ocean had been, 

 chemically and physically, sufficiently near its present status 

 to permit the deposition of vast beds of limestone, and the 

 existence of vegetable and, probably, animal life. It is simply 

 impossible to believe that the action of such a sea upon the 

 land — for land must have existed during this period, and, 

 probably, in masses of continental extent — did not result then 

 as now in the formation of conglomerate, sandstone, and slate. 

 As before stated, the slates, little altered, now appear as 

 argillite and mica slate ; the sandstones are reasonably ac- 

 counted for in the form of quartzites and laminated gneisses, 

 and, probably, some of the finer-grained schistose gneisses ; but 

 the conglomerates, which, with the rare exceptions noted, 

 cannot now be found, offer greater difficulty, unless we admit 

 that, if not the only, they have been, through their metamor- 



