REPORT OF TEE CHIEF ASTRONOMER 783 



SESSIONAL PAPER No. 25a 



So great is the probability of this hypothesis that we may fairly regard 

 the recurrences of augite andesite as so many recurrences of originally basaltic 

 magma. The corresponding large addition to the number and size of the bodies 

 •which represent eruptions of basaltic magma, both in the Forty-ninth Parallel 

 region and throughout the world, greatly limits the total volume of erupted 

 magma which has not been either basaltic or granitic (or granodioritic) in 

 composition. Perhaps less than one per cent of the world's eruptive magmatic 

 bodies, reckoned as to their probable volumes, have had chemical compositions 

 different from granitic, granodioritic, or basaltic magmas. In any case it is only 

 a very small proportional volume of igneous rocks which need explanation as 

 other than crystallized primary basalt, direct differentiates of primary basalt, or 

 granitic differentiates. 



Syntectics. — Since the primary acid shell is not exposed, the theory demands 

 that besides primary basalt and its own differentiates all the other igneous rocks 

 of the Boundary section are solidified syntectics or solidified derivatives of 

 syntectics. This most difficult side of the theory's application has been approached 

 at many points in the foregoing chapters. In the present summary only the 

 more noteworthy considerations need again be mentioned. 



There is an obvious preliminary step to be taken before a full discussion 

 of assimilation is possible. For each magmatic body we should, ideally, know 

 the composition of its country-rock on all contacts, roof, walls, and bottom — if 

 there is a bottom. For stocks and batholiths we have little or no direct informa- 

 tion concerning the nature of the walls for miles below the deepest valley which 

 erosion has carved in the intrusive. If the roof is still largely preserved, the 

 walls are effectively concealed. If erosion has removed the roof entirely, it may 

 be impossible to know exactly of what rocks it consisted. Since stoping takes 

 place on both walls and roof, the knowledge of the petrographic nature of both 

 is essential to an understanding of the product of abyssal assimilation. In each 

 case, the geologist can see only the uppermost part of a batholith, and, in 

 general, he is compelled to regard his field observations as confined nearly to 

 one level in that part. Only indirectly, therefore, can he get ideas as to the 

 form and size of the body and as to the character of the total contact-surface 

 on which stoping and marginal assimilation have held sway. 



One of the principal data for the petrogenic discussion is thus often 

 impossible of full attainment. It can only be found, even qualitatively, after a 

 thorough field study of the invaded formation. Largely for lack of such 

 observations a multitude of petrographic papers are almost useless to the student 

 of petrogeny. Yet more serious is the error of many petrogenists who have 

 decided on the magmatic happenings in batholiths and stocks simply from the 

 chemical relations at visible contacts. This fundamental mistake has been 

 made in the name of ' the scientific method,' which forbids ' speculation ' and 

 leaves the earth's interior ' to the poets,' but it is beginning to show its true 

 character as a tradition which has done much to retard the advance of petrogenj 

 for a generation. 



