R. D. Irving — Is there a Huronian Group f 213 



subordinate strata — each entitled by its thickness and petro- 

 graphical distinctness to be classed with any of the " form- 

 ational " subdivisions of any of the Palaeozoic groups — combine 

 to give it this position. But it remains to be considered 

 whether there is equally strong warrant for believing this series 

 to be structurally, and consequently chronologically, separable 

 from the adjoining portions of the so-called Archaean. Is it 

 not possible that we are merely setting off arbitrarily a set of 

 rocks which is really only part and parcel of the whole great 

 Archaean complex, and is not separable by any genuine line of 

 demarkation ? 



Before attempting to reply to this part of the problem it will 

 be well to understand what sort of evidence that must be 

 whose production will establish a degree of chronological sepa- 

 rateness sufficient to warrant the ranking of this original 

 Huronian as a Group. Such a degree of separateness, then, 

 can only be established when we have in hand good reason for 

 a belief in the subjection of the underlying rocks, as a land 

 surface, to a long-continued atmospheric erosion, before the 

 deposition upon them of the detrital series whose group rank it 

 is desired to establish. In the cases of discordances between 

 groups higher in the geological scale than that we are now con- 

 sidering, the rocks beloiv as well as those above the discordance 

 are of unquestioned sedimentary origin ; so that in such cases 

 we have indicated two periods of submergence, separated from 

 one another by one of disturbance, emergence and erosion. But 

 this accepted aqueous origin for the lower rocks is not essen- 

 tial. We may have equally good reason for a belief in a lengthy 

 intervening period when the lower rocks are crystalline schists, 

 as to whose origin there may be question, or sometimes even 

 when they are eruptives. In the former case the foliation, 

 schistose structure, and general crystalline character of the 

 lower rocks, when contrasted with the absence of such charac- 

 teristics in the higher, indicate the subjection of the former to 

 long-continued and deep-seated processes of alteration, what- 

 ever the first origin, and consequently to profound erosion, 

 ' before the deposition upon them of the overlying detritals. 

 Fragments of the lower crystalline rocks occurring in the 

 higher detritals, particularly at the contacts of the two, serve 

 greatly to strengthen such a conclusion ; since these must be 

 taken as a final demonstration of the production of the schist- 

 osity and foliation prior to the long-continued period of 

 denudation which in turn must have been accomplished before 

 the detrital deposition began. Somewhat similar reasoning will 

 serve even if the lower rocks are of non-schistose, plutonic kinds 

 ("tiefen Ge3teine" of Eosenbusch), since these must have cooled 

 at great depths, and must have been deeply denuded before the 



