REPORT OF TEE CHIEF ASTRONOMER 757 



SESSIONAL PAPER No. 25a 



the facts of the field but it is incomparably more economical of the heat postula- 

 ted for the work of batholithic replacement than is the theory of pure marginal 

 assimilation. Melting and marginal assimilation of country-rock takes place in 

 the initial, superheated condition of a basaltic injection but must be regarded 

 as always subordinate in replacement efficiency to stoping and abyssal assimila- 

 tion. 



Existence of Basic Stocks and Batholiths. — Finally, the fact that some large 

 bodies of plutonic rocks are basic has been advanced as an objection against the 

 idea of stoping.* This fact early impressed itself on the present writer and led 

 to his reviewing the geological literature to determine, if possible, the number, 

 distribution, and age of these bodies. It was found that most of those which 

 appear to have batholithic development on a large scale are of pre-Cambrian 

 age and are chiefly anorthosite intrusions. In the American Journal of Science, 

 vol. 20, 1905, p. 216, the guarded suggestion was made that the anorthosites of 

 Canada and the Adirondack mountains are so basic because of the absorption 

 of crystalline limestones. On maturer consideration this suggestion seems 

 inadequate and a more general explanation must be sought. 



Adams describes the great anorthosite mass of Morin, Quebec, as genetically 

 associated with an adjacent gabbro body of batholithic size.f The one is either 

 a differentiate from the other or both are expressions of a common basic magma. 

 The latter seems the more probable relation. In fact, both bodies appear to 

 represent the crystallized products of a magma allied to, if not identical with, 

 the primary basaltic magma which has been the source of the heat in post- 

 Archean batholithic intrusions. 



The conditions of intrusion for these ' upper Laurentian ' masses seem to 

 have differed from those typically represented in the post-Cambrian batholiths. 

 The latter have been developed under heavy geosynclinal covers which have entailed 

 considerable superheat in the basaltic substratum. It is not impossible that 

 the ' upper Laurentian ' basic magmas, already cooled nearly to the solidification- 

 point, were injected into the then thinner crust, or warped up with it, during 

 crustal disturbance. Lacking superheat these magmas lacked assimilating 

 power and, consequently, did not become acidified. 



In favour of the conception that these magmas were near the solidification- 

 point at the time of their intrusion, is the fact that the anorthosites often show 

 primary banding and are most extraordinarily granulated, as if by dynamic force 

 which acted on the congealing mass near the close of the intrusion-period. Con- 

 cerning the granulation Adams writes: — 



' There are no lines of shearing with accompanying chemical changes, 

 but a breaking up of the constituents throughout the whole mass, though 

 in some places this has progressed much further than in others, unaccom- 

 panied by any alteration of augite or hypersthene to hornblende, or of plagio- 

 clase to saussurite; these minerals though prone to such alteration under 



* W. Cross in Science, Vol. 25, 1907, p. 620. 

 t F. D. Adams, Canadian Record of Science, 1894-5. 

 25a — vol. iii — 49J 



