THE ARCHEOZOIC ERA. 157 



would seem that there should be somewhat wide-spread evidence of the 

 fact in the structure of the system. So thick a series of sediments 

 would surely have been made up of layers of varying physical and 

 chemical composition. From such a series, no matter how completely 

 metamorphosed, it is not probable that the original distinctions of con- 

 stitution w T ould have been generally obliterated; but in most of the 

 Archean, no such succession of strata as would be expected if the rocks 

 were dominantly meta-sedimentary exists. This point has special 

 force when it is remembered that it holds not for one region only, but 

 for each of the many regions, some of them of great extent, where the 

 Archean rocks have been studied. It would appear that such effective 

 and wide-spread obliteration of the characteristics of sedimentary rocks 

 could only have been brought about by their fusion; and in this case 

 the product, on solidifying, should no longer be regarded as metamorphic 

 rock, but as igneous rock instead. 



A special phase of this hypothesis makes the Archean the altered 

 product of a chemical precipitate from the original ocean. This 

 hypothesis never met with general favor, and has rarely been seri- 

 ously entertained. 1 



2. A second hypothesis which has been advanced is that the Archean, 

 at least locally, consists of igneous rock produced by the fusion of 

 deeply buried sediments. 2 If this hypothesis were true, the igneous 

 rocks produced by the fusion of sediments should grade up into sedi- 

 mentary rocks above; they should not be separated from them, as 

 they generally are, by an erosion unconformity. Otherwise it would 

 be necessary to suppose that the lower igneous series was fused long 

 before the upper series was deposited, and that the beds which originally 

 overlay the igneous series, and which were not fused, were subsequently 

 removed by erosion before the deposition of the existing superior, 

 unconformable, sedimentary series. While this sequence of events 

 could not be disproved for certain localities, it is not probable that 

 it represents the general history of most regions where the Archean 

 occurs. 



3. A third hypothesis is that the Archean reoresents igneous rock 



1 Hunt, The Taconic Question in Geology and the Origin of Crystalline Rocks 

 Trans. Roy. Soc. Canada, Vols. I and II. The untenability of this hypothesis was 

 shown by Irving, 7th Ann. Rept. U. S. Geol. Surv., p. 383. 



2 Lawson, Ann. Rept. Canadian Geol. Surv., 1887. 



