702 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Primary Acid Shell of the Earth. 



The natural supposition that a once molten earth would have become strati- 

 fied through density has been made probable by the more recent studies of 

 silicate melts and of natural magmas. Irrespective of pressure, the primitive 

 differentiation of earth-magma would give liquid layers of absolute density 

 increasing with depth. The increase might be gradual or it might occur in 

 relatively sharp changes from layer to layer, each of which was immiscible with 

 its neighbour at the ruling temperature. According to the second view each 

 layer would be expected to have a fairly uniform composition. If one of the layers 

 was basaltic, as implied in the preceding section, the overlying layers were lighter 

 and presumably more acid than basalt. We may now briefly examine the view 

 that the uppermost primary layer, or earth-shell, was granitic in composition. 



Every worker in the pre-Cambrian sediments is struck with the pre- 

 dominance of quartz fragments. Granites or gneisses are certainly the principal 

 sources of such silicious material. When we reflect that the earliest known 

 sediments are thus quartzose; that the total thickness of the pre-Cambrian 

 quartzose sediments, as measured in eastern Canada, British Columbia, Finland, 

 and elsewhere, runs into tens of thousands of feet, we may be sure that the lands 

 prevailing throughout most or all of recorded pre-Cambrian time were of grani- 

 tic (gneissic) composition. Such terranes are exposed on an enormous scale in 

 a few parts of the earth and are fairly to be understood as forming the greater 

 part of the present continental plateaus. The film of sedimentary rocks on these 

 plateaus averages so thin that no essential doubt can remain as to the general 

 character of the surface shell through which Paleozoic and later igneous 

 eruptions have taken place. A rough quantitative study of available maps shows 

 that this shell is, on the average, everywhere of granitic composition. Two lines 

 of evidence thus converge to the belief that from the earliest time recorded in 

 the pre-Cambrian sediments to the time of the great Cambrian overlap, the 

 surface rocks of the globe were dominantly granitic (or gneissic). Can we go 

 further and hold that the first stable shell formed on the cooling globe was of 

 similar granitic composition? The speculative attempt to answer the question 

 has some value. 



Dutton suggested that the visible granites, gneisses, syenites, etc., were 

 produced by the remelting of sediments derived from a general and primordial 

 basaltic shell, implying that the lands of most of pre-Cambrian time were 

 basaltic. He writes: — 'Chemical considerations of a cogent character lead up to 

 the inference that primordial magma ought to possess a constitution similar to 

 rocks of the basaltic group, though perhaps somewhat less ferruginous (?), and 

 that it should be nearly homogeneous.' And again: — 'We know of no natural 

 processes capable of separating the more acid parts of such a magma except the 

 chemistry of the atmosphere acting at temperatures far below the melting- 

 points of the silicates. We have the results of that process in the quartzites, 

 granites, gneisses, and syenites among the silicious rocks ; and the limestones and 



