20 
MUSEUM BULLETIN NO. 30 . 
Triple Points. 
It is of interest to note the occurrence in the gabbro of two 
fairly definite limits, which may be termed “ triple points ” for 
want of a better name. As already described, the gabbro is not 
of uniform composition throughout, but includes types ranging 
from highly basic to extremely siliceous. This change is accom- 
panied by a corresponding change in the composition of the feld- 
spar from anorthite in the most basic varieties to albite in the 
most siliceous. In the most basic types olivine forms approxi- 
mately 25 per cent of the rock, and is accompanied by pyroxene 
and anorthite or bytownite. The proportion of olivine becomes 
less and less in the rocks characterized by progressively more 
sodic feldspar, until it disappears altogether when the feldspar 
becomes Ab 40 An 60 . The point at which olivine ceases to appear 
is the first “triple point,” so called because it apparently marks the 
limit of the equilibrium of olivine, pyroxene, and feldspar in an 
increasingly sodic series of magmas. It is true that many rocks 
with feldspar more calcic than Ab 40 An S0 do not contain olivine, 
but this is ascribed to the removal from such rocks of the olivine 
crystals, by sinking, after the partial crystallization of the rocks. 
The second " triple point ” observed is identical or nearly so 
with that found by W, H. Collins in the Gowganda diabases. 1 
At this point feldspar, pyroxene, hornblende or biotite, and free 
quartz are in equilibrium, and the feldspar has the composition 
Ab e5 An 35 . Rocks containing feldspar more calcic than this are 
composed of pyroxene and feldspar with accessory titanium and 
iron combined as titaniferous magnetite. Rocks with more sodic 
feldspar contain in addition hornblende and biotite (Collins 
found biotite only), with free quartz, and accessory iron and 
titanium take the forms of titanite and magnetite. 
Emphasis may be laid on the fact that the sudden mineralo- 
gical changes at the “ triple points ” are not accompanied by any 
sudden change in chemical composition. On the contrary, all 
the evidence indicates that the chemical changes are gradual and 
smoothly progressive. Collins found a similar condition to pre- 
vail. 
i Collins, W. H., Geol. Sunr., Can., Mem. 33, 1933, p. 80. 
