MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 345 
Further we include a series of rhyolites, dacites, spherulites, and 
quartz-porphyries, in which phenocrysts of feldspar (orthoclase or 
plagioclase) as well as of quartz usually appear. Gl.=glass basis; 
Grm.=groundmass; Sph.=Spherulite. 
TABLE VII 
PERCENT- SPHERULITIC 
AGS OS DacITES RHYOLITES AGES 
Sph. Grm. | Grm. |Sph.and GI. | Gl. Sph. Gl. Gl. Sph. Gl. 
HOG) 4... . SiO, 174.6 |75.07|/72.5 Wha N7BcO 71.4 
76.5 72.8 
Gl., Grm., | {SiO, [76.05*|74.96/77.51| 72-7 |72-6 |74.5|72-5]74.6 |73.7|70-7 
or Sph...| |H,O | 22 iS | woz ©. || ASS TA ALAl uO | BoA 8.7 
* Only ro.24 per cent Al.O;. Somewhat decomposed. 
{ Only 1.52 per cent Al.0;. Somewhat decomposed. 
Percent- Spherulitic Rocks with Quartz-Porphyries with 
age of |Spherulites and Glass Basis Groundmass 
Sollee oe as. neo: 73-2 174-4 175.4 |73-4|Qu-porph.. .| 72.0 he, TAP ERs 
WW, 172.4 |72.7 |73-9 |73-1 74-4]74-4](70.05 
Cl... ron Ae glee la onl AS a 
The glass basis, or the groundmass, in the Bonnin rocks 
consequently shows: 
a) In rocks with about 60 per cent SiO,, a sometimes very 
considerable increase in the percentage of SiO, (for example, from 
58.1 to 70.8 per cent SiO.) and in rocks with about 65 or 65-70 
per cent SiO, a smaller increase, though in undecomposed state 
not above 75 per cent SiO.. 
b) In rocks with about 73-75 per cent SiO, we find, on the 
other hand, about the same percentage of SiQ, in the glass basis or 
groundmass es in the entire rock. The analyses show some slight 
variations, partly in one and partly in the other direction. But 
this is certainly caused in some cases only by slight inaccuracies in 
the relatively old analyses, and in others by the groundmass and 
especially the glass basis (probably without exception) being a little 
decomposed, as shown by a little H,O. 
These law-governed relations, which are established by nu- 
merous analyses, may depend on the fact that in a magma 
consisting chiefly of Qu and Or, Ab and An components, with a 
