GABBROS OF EAST SOOKE AND ROCKY POINT. 
39 
General Discussion of Figure 1. 
The mode of construction and interpretation of Figure 1 
have already been described, and it has been shown that to deter- 
mine absolute gains and losses during an alteration the weight, 
the volume, or the arhount of some one constituent must be 
known to have remained constant, or, if not constant, the extent 
of its change must be known. 
In the present case no one constituent can be assumed to 
have remained constant, as all were probably very soluble in the 
hot solutions. If alumina, the most insoluble oxide, were sup- 
FigTjre 1 . Diagram illustrating hornblendization and aplitization. The hornblendic alter- 
ation is indicated by the solid line, the aplitic alteration by the broken line. 
posed constant throughout, the curves show that a large increase 
both in weight and volume must have taken place, of which there 
is no field evidence. The aplite is approximately 10 to 15 per 
cent lighter, the hornblendite about the same amount heavier, 
volume for volume, than the gabbro. If, therefore, the weight 
remained constant throughout the alteration, it must have been 
accompanied in the aplite by increase, in the hornblendite by 
decrease of volume. There was no evidence observed in the field 
that either occurred. The most probable assumption appears 
to be that volume remained constant or nearly so. If so, the 
zero point for the hornblendite curve would lie between the 
vertical lines 1 10-1 1 5, for the aplite curve between verticals 85-90. 
