THE MONTEREGIA N HILL S 2 8 1 



in an area which has undergone much more extensive denuda- 

 tion since the time of the intrusion than in the cases above men- 

 tioned, and as a consequence of this the fragmental material 

 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 differ- 

 entiation is very complete, but in which the magma did not 

 remain at rest, but was not long prior to final consolidation, 



imD^ 



Troi-^slt'tonttl Rock 



Fig. 7. — Diagrammatic cross-section of Mount Johnson, showing the relation of 

 the several rock types. 



moving upward, it seems improbable that the marked differen- 

 tiation 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, rep- 

 resented 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 



