REPORT OF THE CHIEF ASTRONOMER 749 



SESSIONAL PAPER No. 25a 



are, we seem justified in concluding that the earth's crust is now, as a whole, 

 ie stable flotation.* 



It may have been entirely different in pre-Keewatin (earliest Archean time, 

 when the superficial, acid couche of the primitive earth began to solidify. Then 

 foundering may have taken place, as Kelvin imagined, and the early formed 

 crusts could have sunk half a score of miles or more until they met the denser 

 couche below. Possibly some of the complexity of the pre-Cambrian formation 

 may be referable to this unstable condition of the early crust. Already in 

 Keewatin times the acid shell was solidified and was then penetrated by basaltic 

 injections which reached the surface, forming the heavy masses of greenstones 

 belonging to that period. Since then the crust has remained essentially coherent, 

 and through it the primary basalt has, at many times and places, been erupted. 

 It is, however, quite possible that the lack of system among the axes of the 

 Laurentian batholiths and the abundance of those batholiths are both explained 

 by the thinness and weakness of the crust in post-Keewatin and pre-Cambrian 

 time. 



For Paleozoic and later batholiths there is a well-defined law that they 

 have penetrated the crust only on the sites of folded geosynclinals, and that the 

 larger batholithic axes are usually arranged parallel to the respective geosyn- 

 clinal and mountain-range axes. 



In other words, the intrusion history of the globe may be conceived as 

 divisible into three epochs : the first being that in which the outer primary 

 shell was becoming stable through successive solidifications and founderings, 

 the second being the post-Keewatin (Laurentian) epoch of very general inter- 

 action between the fluid basaltic substratum and acid crust, without extensive 

 founderings but with development of many large, irregularly occurring batho- 

 liths; the third, a period of the localization of batholiths in certain mountain- 

 built belts, where alone there seems, in this third period, to have occurred the 

 injection of molten magma in masses of batholithic size — rarely, if ever, accom- 

 panied by wholesale foundering. f 



Again, granting the hypothesis that a visible post- Archean batholith is the 

 acidified, upper portion of a basaltic body originally injected to a level less than 

 about six or eight miles from the earth's surface (perhaps the level of no 

 strain), it is not difficult to see that extensive foundering may be impossible. 

 Only after some differentiation or acidification of the primary magma would 

 any part of it become less dense than the average roof-rock. Xenoliths of the 

 heavier gneisses and schists would, however, sink. When dissolved in the pri- 

 mary magma their material — added to that dissolved along the main contact — 

 would lower the density and inaugurate the stage of general stoping. Only 



• For a further discussion of this point see Amer. Jour. Science, Vol. 22, 1906, p. 

 201. 



t Is it certain that the rhyolite plateau of the Yellowstone Park is not the site of 

 partial foundering? The vastness of the formation suggests, in any case, that the 

 youngest of the American batholiths lies but little below the surface in the Park. The 

 geyser heat is probably derived from this still cooling batholith. Since this report 

 was sent to Ottawa for publication, the writer has issued a fuller statement of this 

 suggestion (Proc. Amer. Acad. Arts and Sciences, Vol. 47, 1911, p. GO). 



