REPORT OF THE CHIEF ASTRONOMER 24© 



SESSIONAL PAPER No. 25a 



tion of sill A. The zonal character of the four individual sills is clearly due to 

 gravitative adjustment. 



The same principle has probably controlled the rough system implied in the 

 succession of average rock-densities of sills D, C, and A, as illustrated in the 

 following table: — 



Sill. Approximate mean densities, 



A . . 2-83 



C 2-91 



D 2-98 



The layer of quartzite separating sills C and D is only 75 feet thick. It 

 is entirely possible that this layer is wedge-shaped, or else was penetrated by 

 one or more connecting dikes. By such means B and C might have been in 

 magmatic communication. We may imagine a partial differentiation within 

 this larger chamber, whereby sill C became more acid, on the average, than 

 sill D. Continued differentiation by gravity, within the partially separated 

 masses C and D, led to the observable stratification of eacb. It is not impos- 

 sible that all four sills were similarly connected in a common (sill-like) magma 

 chamber, from which each visible sill was a kind of great, flat apophysis. 



On the other hand, these sills may not have been of exactly contemporaneous 

 intrusion. A large part of the magma now represented in the rock of sill A 

 may have formerly rested in the chamber of sill B. Since then, after partial 

 differentiation, that part of the magma may have broken through the roof of 

 chamber B to the new horizon now occupied by sill A, where continued differ- 

 entiation produced the actual zonal arrangement. Similarly, sills A, B and C 

 may have been apophysal from the great sill D. One cause for such successive 

 injections may be found in the enormous gas-pressure generated by the assimi- 

 lation of moist quartzite — a tension amply sufficient, under certain conditions, 

 to fissure the roof of the slightly older chamber and cause the rise of the 

 magma to the higher horizons of the existing upper sills. (See Figure 16.) 



In spite of the relative complexity of the whole system, we may conclude 

 that gravitative differentiation is clearly the dominant process in developing 

 the zonal structures of the Moyie sill group. 



It is hardly necessary to dwell on the chemical side of the differentiation. 

 It was probably founded on the limited miscibility of gabbro and secondary 

 magma at the low temperature immediately preceding crystallization. The 

 magma was not quite the chemical equivalent of the invaded sediments. Eacb 

 of the two granite types contains more ferrous iron and lime than the average 

 quartzite. The hornblende-bearing granite is clearly more ferromagnesian and 

 calcic than the sediment. However, the total volume of the gabbro in the sill 

 system is so great that its average original composition was not essentially 

 affected through the transfer of the extra lime, iron oxides, and magnesia to 

 the granite zones. 



Similar and Analogous Cases. — The writer's explanation of the Jaoyie sills 

 has been greatly strengthened by the discovery of similar features in other 



