IGNEOUS ROCKS. 
61 
lime percentage is accounted for by the presence of infiltrated calcite. 
In analyses C, D, E, and F the low magnesia explains the absence 
of hornblende and diopside and the lowering of the biotite percentage. 
The mineral compositions calculated from the analyses (p. 58) 
are in general similar to those observed, yet on account of fine- 
grained or amorphous groundmass the mineral composition a^s deter- 
mined from thin sections could only be approximate and in some 
instances does not correspond well with the calculated modes, the 
greatest discrepancy being in the relative amounts of quartz, ortho- 
clase, and lime-soda feldspar. Where the groundmass is crypto- 
crystalline, the mineral composition is based on the phenocrysts. In 
the dacite and andesites these form a large percentage of the mass of 
the rock, while in the early trachytes, the early rhyolite, and the 
latest trachyte they are subordinate in amount. Sillimanite and 
wollastonite were not seen in the rocks, but were added to take up 
surplus alumina and lime. 
RELATIONS OF LACCOLITHS AND EFFUSIVES IN GENESIS. 
The question naturally arises whether or not the intrusive andesite 
and the effusives came from the same reservoir. The chemical com- 
positions of the laccolithic rocks and the different flows show but a 
small range. With very little differentiation all the different phases 
may have originated from the same parent magma. It is certain, 
however, that the laccoliths did not act as vents through which the 
outpouring of the effusives took place. The andesite had been intruded, 
solidified, and eroded when the lavas were poured out over the eroded 
edges of the uplifted sediments. That the lavas came to the surface 
through the laccoliths after their solidification and erosion does not 
seem likely, since the andesite areas show no dikes or stocks, unless 
the ore veins be so called. Neither are there stocks or dikes else- 
where in the district through which the eruptions might have occurred ; 
hence we are driven outside of the area for the source of the effusives. 
The following table shows an approximation of the average chem- 
ical composition of the lavas and andesite, obtained by averaging the 
preceding analyses. The general similarity of composition indicates 
that both may have come from the same reservoir. 
Average chemical composition of laccoliths and effusives. 
Si0 2 . 
AI2O3 
Fe 2 3 
FeO. 
MgO. 
CaO. 
Na.aO 
Laccoliths. 
Effusives. 
64.46 
66.42 
13.60 
15.41 
2.85 
3.48 
1.80 
.96 
2.59 
1.34 
4.65 
3.40 
1.90 
1.86 
K 2 
H 2 o 
PaOi 
BaO 
Laccoliths. Effusives. 
5.20 
2.21 
.18 
. 11 
99.55 
.95 
. L9 
.09 
100.09 
