466 HENRY S. WASHINGTON 
purely ferrous olivine, fayalite’ in the granite, and the presence 
of the MgO-free ‘“ glaucophane”’ molecule in the blue horn- 
blendes? of the region. 
Discussion of oxide ratios—In Table IJ are given, below the 
molecular amounts, several oxide ratios, which we may next 
examine, since they serve to differentiate the rocks of the region 
into groups. 
Na,O 
EO 
_have seen, though often closely approaching it. A striking fea- 
ture is that in ten or eleven cases it is either exactly or very near 
a’ whole number; tel, 221, 3:1, 01 O21. Ini tenvothersent 
closely approximates to multiples of a half; 1%:1, 2%:1, 
314:°1, or 6%:\1. In only three or tour’ cases does atidities 
more than about .12 from such figures. There seems to be a 
tendency for Na,O and K,O to exist in stoichiometric ratios 
with respect to each other. The same is true elsewhere, notably 
in the Christiania region, where Brégger3 connects it with the 
tendency of the alkalis to such ratios in nepheline and soda- 
orthoclase. Such ratios, however, in igneous rocks are by no 
This ratio is constantly greater than unity, as we 
means general. 
When we examine this ratio in the various rocks we see that 
it is characteristic of certain rock groups. In the granite, the 
quartz-syenite-porphyry and the quartz-syenites it is quite con- 
stant, varying only from 1.09 to 1.17. In the aplite dike it is a 
little higher, about 1.5. Inthe keratophyre and rhyolite it is still 
higher, 1.40 and 2.20. As we go toward the basic end the 
ratio increases, being 1.7 and 2.1 in the pulaskites, 2.2, 2.5, and 
3.6 in the sdlvsbergites and biotite-tinguaite, and 3.0 in the 
foyaites. In the basic rocks, from diorite down, the ratios are 
constantly high, varying from 2.6 to 6.6, and is also high, 6, 
in the analcite-tinguaite. 
t PENFIELD and ForBES: Am. Jour. Sci. (IV), I, 129, 1896. 
2H. S. WASHINGTON: Am. Jour. Sci. (IV), VI, 179, 1898. 
3BrROGGER: Eruptivgesteine der Kristianiagebietes. I, 165, 1894, and III, 
249, 1897. 
