PETROGRAPHY. 
77 
hypersthene, was assigned to the augite molecule, and the silica left after these read- 
justments and the calculation of orthoclase and anorthite was divided between albite 
and nephelite molecules in the usual way. The soda-lime feldspar was assumed to 
have the composition Ab 3 An 4 , thus leaving a little soda for the alkali-feldspar. The 
results of this calculation are: 
Soda-orthoclase, OrjAbj 52.1 
Labradorite, Ab 3 An 4 17.8 
Nephelite 7.8 
Augite. 
17-7 
Biotite 1.7 
Magnetite 2.4 
Apatite 0.5 
These figures agree with the appearance of the section. Thus the measured 
amount of the two feldspars and the nephelite base was about 77 per cent, while the 
calculated total for these is 77.7. The calculated amount of augite is slightly 
greater than the measured, and it is possible that some of the very small augites of 
the groundmass have been overlooked, but the divergence is not large. From treat- 
ment of the section with acid and fuchsine it would seem that the calculated amount 
of nephelite corresponds closely to the actual. 
Taking up the Arso rock the same difficulites meet us, as regards the feldspars 
and glass cement, when it is attempted to estimate the mode by Rosiwal's method. 
But here, likewise, the amounts of leucite. augite, and olivine could be measured 
with a reasonable decree of certainty. The results are, for leucite 1.6 per cent, for 
augite ii per cent, and for olivine 2.1 per cent, the total amount of phenocrystic and 
groundmass feldspars and glass being 84, and that of ores about 1.5 per cent. 
Although the texture is somewhat hyaline and the mode therefore is indeter- 
minable, strictly speaking, the amount of glass is so small that the calculation of the 
mode from the norm may be attempted. For this purpose it was assumed that 
the amount of leucite was that shown optically and that the plagioclase had the 
composition shown below. The result is: 
Soda-orthoclase, Or 4 Ab 3 55 . 2 
Andesine, Ab 4 An 3 20.1 
Leucite 1.7 
Nephelite 4.0 
Augite 13.1 
Olivine 1.7 
Magnetite 3.4 
Apatite 0.8 
From the somewhat greater amounts of augite and magnetite shown by the 
calculated mode, and the presence of nephelite, it is clear that the glass cement 
is essentially a nephelite in composition (which is confirmed by the fact that it gel- 
atinizes readily with acids), and that it carries considerable augite and magnetite, 
either as the fine, dusty microlites, or uncrystallized as the brownish coloring 
material. The olivine shown in the mode closely approximates to that of the norm. 
It is of interest to note here that, instead of the small amount of modal biotite 
