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more extensive denudation since the time of the intrusion 
than in the cases above mentioned, 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 oc 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 moving upward 
Quarry in andose, Mount Johnson, showing vertical flow structure (on right). 
long prior to final consolidation, it seems improbable that 
the marked differentiation of the magma into the several 
varieties 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 
