212 
IDDINGS. 
occurring at Electric Peak. They vary quite irregularly for 
a gradual change in the silica. The alumina varies rapidly 
and retains a high position. The alkalies gradually in¬ 
crease with the silica, the soda molecules being more than 
twice as numerous as those of potash. Magnesia experiences 
the greatest variation which is strikingly opposed to that 
of the alumina. The lime is less regular than in the Elec¬ 
tric Peak group. The two oxides of iron are remarkably 
uniform in their reciprocal relations. The magnesia, lime 
and iron diminish with the increase of silica, while the 
alkalies increase, and the alumina decreases slightly. In 
this group of rocks also the oxidation of the iron bears a 
noticeable relation to the presence of hornblende and biotite 
and magnetite. 
It is evident from a study of the analyses that the chemi¬ 
cal variations in this group of volcanic rocks are the same 
in character and extent as those in the intrusive rocks of 
Electric Peak. Further, it is shown that the variations be¬ 
tween similar varieties of andesite—such as those between 
different pyroxene-andesites—are as great, and in some cases 
greater than the variations between varieties of andesites 
which are distinguished mineralogically from one another. 
Thus 219 and 221 are pyroxene-andesites without horn¬ 
blende ; 214 and 217 are hornblende-pyroxene-andesites, 
while 694 is a hornblende-andesite. It is not possible to 
point to any chemical character of these rocks as distinctive 
of this mineral variation, with the exception of the oxidation 
of the iron, which, though slight, is an important one; for 
it undoubtedly relates to forces which did not alter the fun¬ 
damental relation between the bases in the magma, but 
simply modified it by changing the oxidation of one of them 
The last four analyses are of hornblende-mica-andesites 
and dacites. The chemical variations between them are 
quite as pronounced as those between the more basic mem¬ 
bers of the series, without there being the corresponding dif¬ 
ferences between the kinds of ferromagnesian silicates so far 
as it can be detected microscopically. They all carry horn- 
