REPORT OF THE CHIEF ASTRONOMER 253 



SESSIONAL PAPER No. 25a 



it is necessary that, more or less completely, the petrologist shall know his 

 magma chamber as the chemist or metallurgist knows his crucible. No student 

 of fused slags can obtain safe results from the profoundest examination of 

 merely one surface or one section of the fused product. He must think in 

 three dimensions. In the same way, the petrologist attempting to unravel the 

 complex history of a magma chamber, should, ideally, know its general shape, 

 size, and contents, as well as the method by which the chamber has been opened 

 within the earth's crust. Until these conditions are fulfilled his problem of 

 rock-genesis through magmatic differentiation remains wholly or in part 

 unsolved. 



The geologist knows how hard those conditions are. He is dependent upon 

 erosion's rendering his contact accessible; yet erosion destroys surfaces of 

 eontact. He can find no bottom to the chamber of stock or of batholith, though 

 large-scale differentiation is most commonly evinced in stocks and batholiths. 

 It is not to be wondered at that, notwithstanding the great number of des- 

 cribed instances of magmatic differentiation, the phenomenon itself is so little 

 understood or that the origin of the igneous rocks is still shrouded in the mists 

 of hypothesis. In view of the difficulties surrounding the study, the discovery 

 of single cases where the requisite field conditions are tolerably well fulfilled, 

 merits special statement. Descriptions of bodies differentiated in chambers 

 of known form are in the highest degree rare. Nevertheless, it is precisely in 

 the light of these rare cases that the laws of differentiation can be most intelli- 

 gently discussed. 



Such instances are discussed in this chapter, in which have been described 

 exceptionally clear examples of differentiation within magmatic chambers, the 

 •crystallized contents of which can now be examined from top to bottom. The 

 form and geological relations of the chambers are sufficiently well determined 

 to serve for the discussion of the magmatic problem. The general nature of 

 the magma whence differentiation has evolved the existing igneous rocks is 

 believed to be deductible from the field and chemical relations in each case. 

 The compound magmas were themselves derived, owing their composition to 

 the digestion or solution of acid sedimentary rocks in original gabbro magmas. 

 Finally, the facts seem indisputable as to the nature of the method by which 

 the differentiation took place. The actual segregation of the sub-magmas 

 appears to have been directed by gravity, producing simple stratification in the 

 chambers. In each sill the less dense sub-magma of splitting overlies the 

 denser sub-magma of splitting. 



In almost every case the opponents of the assimilation theory have treated 

 of the assimilation as essentially a static phenomenon. Each interpretation 

 of field facts has been phrased in terms of magmatic differentiation versus 

 magmatic assimilation as explaining the eruptive rocks actually seen on the 

 contacts discussed. Nothing seems more probable, however, than that such 

 rocks are often to be referred to the compound process of assimilation accom- 

 panied and followed by differentiation. The chemical composition of an intru- 

 sive rock at a contact of magmatic assimilation is thus not simply the direct 



