244 Canadian Record of Science. 



which fills some, although not all of the necks referred to 

 above, has been entirely swept away. 



In view of the fact, then, that Mount Johnson is a 

 neck or pipe of comparatively small sectional area, in 

 which the differentiation is very complete, but in which 

 the magma did not remain at rest, but was long prior to 

 final consolidation, moving upward, it seems improbable 

 that the marked differentiation of the magma into the 

 several varieties described in this paper took place while 

 the magma was in the pipe itself. The evidence points 

 rather to the differentiation of the mass having already 

 taken place in the reservoir of molten rock beneath, 

 which was tapped by the pipe. If this be the case, it 

 would seem that the upper and more acid portion of the 

 magma, represented by the lighter pulaskite, had collected 

 in the upper portion of the reservoir, and that the essexite 

 formed a lower, more basic, and heavier stratum or part. 

 When the passage to the surface was opened up, the 

 pulaskite would first rise in it and, after a more or less 

 long-continued flow, being followed by the essexite, would 

 be pressed toward the circumference of the pipe, the more 

 basic rock occupying the central portion of the passage, 

 and the most basic variety, originally lower, would be 

 found in the central axis of the neck. The fact that, 

 while the essexite forms the mass of the intrusion, there 

 is a zone of pulaskite about it, would seem to indicate 

 that there had not been at this center of volcanic activity 

 any very protracted outpouring of the essexite, since, had 

 this been the case, it would seem probable that the pipe 

 would have in time been cleared of the earlier pulaskite 

 magma. 



The interesting question of the succession of the 

 eruption of the several magmas in this petrographical 

 province, as well as the causes of their differentiation, can 

 be more profitably discussed when the other centers of 

 eruption have been more thoroughly studied. It is 

 interesting to note the cumulative evidence in favor of 



