CURRENT PRE- CAMBRIAN LITERA TURE 1 93 



local character of the conglomerates, have considered the series includ- 

 ing them as a unit, and, regarding the hiatus at the top of the series 

 as much more profound than that at the bottom, have included the 

 series in the Algonkian, rather than in the Cambrian. Winchell and 

 many of the Canadian geologists, on the other hand, have recognized 

 but one conglomerate within the Keweenawan, and, giving this undue 

 significance, have extended the Cambrian downward to this conglomer- 

 ate, thereby including in the Cambrian the profound unconformity 

 separating the Keweenawan series and the fossil-bearing Cambrian. 

 If Winchell's reasoning be carried to its logical conclusion, wherever a 

 conglomerate is found within the Keweenawan, the series must there 

 be divided, and in place of one series, we shall have many series, a 

 division which cannot be considered reasonable. It appears, therefore 

 that Winchell's division of the Keweenawan into two series, on the 

 basis of local conglomerates such as the one described from Taylor's 

 Falls, is purely arbitrary, and furthermore, by referring his upper 

 division to the Cambrian, the profound unconformity known to exist 

 between the true Cambrian and the Keweenawan is practically ignored. 



Winchell 1 discusses the Archean greenstones of Minnesota, which 

 he considers the oldest known rocks, representing the original crust of 

 the earth. The greenstones are divisible into iwo parts, one igneous 

 and the other clastic, the latter succeeding the former with a confused, 

 and apparently sometimes conformable superposition, somewhat as 

 surface eruptive rocks might be superposed, in the presence of oceanic 

 action, upon a massive of the same nature at the same place. The 

 clastic portions of the greenstones vary to more siliceous rocks, con- 

 stituting great thicknesses of graywackes, phyllites, and conglomerates, 

 and as such have been converted by widespread metamorphism into 

 mica-schists and gneisses. 



As the Laurentian gneisses and granites cut the schists and sedimen- 

 tary gneisses they are also younger than the bottom greenstones. 



The metamorphic schists and gneisses seem to be representative of 

 the sedimentary portion of the Lower Laurentian of Canada, while the 

 igneous granite and gneisses are as plainly a general parallel of the 

 igneous portion of that series. It follows, therefore, that the Canadian 

 Laurentian is, as a whole, of later date than the greenstones, if the 



. ' The oldest known rock, by N. H. Winchell : Proc. Am. Assoc. Adv. Sci. Vol. 

 LXVII, 1898, pp. 302, 303 (Abstract). 



