OLIVINE-BEARING QUARTZ-MONZONILE FROM KIANDRA. yas \ | 
Thus the slight chemical differences between the two 
rocks have separated them as far as class and order are 
concerned. This is due chiefly to the higher percentages 
of alkalies, and especially of magnesia, in the porphyritic 
rock, which have had the double effect of increasing the 
proportion of femic molecules in the norm, and of decreas- 
ing the amount of silica available for quartz. 
Hypersthene is the dominant pyroxene, just the reverse 
of what is found in the mode. As only a small proportion 
of this normative hypersthene is contained in biotite and 
hornblende, it is evident that a good deal of it must be 
present in the augite of the rock, as was inferred on optical 
grounds, 
Chemically the rocks are different from the normal types 
of monzonites, the iron and magnesia, and perhaps lime, 
being rather higher, and the alkalies rather lower than 
usual. This is brought out by comparison with Broggers 
average monzonite, quoted in column V. 
The rather exceptional composition of the rocks is also 
made plain by the fact that it is difficult to find in Wash- 
ington’s Tables any rocks, belonging to the same subrangs, 
that were at all closely comparable, two of the nearest 
being given in columns III and IV. 
The Cooling-History of the Quartz-Monzonite Magma. 
In the absence of detailed information as to the field- 
occurrence of the different rocks just described, it is per- 
haps rather futile to speculate as to their probable mutual 
relationships. It is perfectly clear, however, that they 
are Closely connected genetically, and that they represent 
portions of the same magma cooled under different con- 
ditions, the monzonite-porplhyry possibly representing a 
relatively rapidly-cooled marginal phase of the quartz- 
monzonite. 
