REPORT OF THE CHIEF ASTRONOMER 247 



SESSIONAL PAPER No. 25a 



Assimilation through Magmatic Vapours. — Again, the influence of mag- 

 matic water and other vapours must be given due weight. The quartzites 

 to-day are not entirely dry rocks. They must have been moister in that early 

 time when the intrusions occurred. From heated roof and floor of each sill, 

 and from each heated xenolith, water vapour must have been injected and forced 

 into the sill magma. The volatile matter contained in assimilated sediment 

 must similarly enter the magma. A large part of such vapour would rise to 

 the roof, and there aid in the solution of the quartzite. Such resurgent vapour 

 must not only lower the solution-point (of temperature) for the roof -rock; it 

 must also specially metamorphose the sediment outside of the magma chamber. 

 It is a fact that the quartzite above each sill seems to be more thoroughly 

 crystallized than the quartzite below the sill. 



Summary of the Arguments for Assimilation. — The facts and deductions 

 bearing on the subject are so numerous that it will be convenient to review 

 them in brief statement. The writer's belief in the principle of assimilation 

 as a partial explanation for the acid zones in the Moyie sills is founded on the 

 following considerations : — 



1. The strong mineralogical and chemical similarity between the biotite 

 granite and the invaded quartzite. 



2. The existence of solution aureoles about the visible xenoliths of quartzite. 



3. The field evidences of superfusioh in the sill gabbro. 



4. The relation between sill-thickness (heat supply) and degree of acidifi- 

 cation. 



5. The necessary recognition of various loci of solution in the sill, namely, 

 at roof and floor, at xenolith contacts, and in the feeding channels below the 

 sills. Resurgent and juvenile vapours, collected at the roof, must tend to 

 hasten solution in that place specially. 



6. The fact that differentiation may partially mask the direct evidence of 

 assimilation. 



7. The existence of many other sills and sill-like intrusions showing similar 

 or analogous relations of gabbroid magma to sediments. Some of these cases 

 will be listed after the nature of the differentiating process at the Moyie sills 

 has been sketched. 



8. The inadequacy of the hypothesis that the various phases of the sills 

 are due only to the pure and simple differentiation of a primary earth-magma. 

 This point is implied in the foregoing argument; it was briefly discussed in 

 the writer's 1905 paper. 



Gravitative Diffieremtiation. — Inspection of Table XIII. and of Figure 15 

 will lead to the conviction that, in sills C, D, and E, the igneous rock is strati- 

 fied. In each of these instances the specific gravity increases from top to bottom 

 of the sill. The same is true of sill A, with the exception of the shell of gab- 

 broid rock next the roof and overlying the biotite granite. An explanation for 

 this exceptional arrangement of zones has been given in the preceding descrip- 



