162 
IDDINGS. 
variable than the alkalies and alumina. For this reason the 
more silicious rocks of this region are less variable in com¬ 
position than the less silicious rocks. 
The process of differentiation has separated the average 
magma into partial magmas, some of which are high in 
silica, with much alumina and alkalies and little magnesia, 
lime, and iron oxide; and into others low in silica, with 
nearly as much alumina and slightly less alkalies, but 
abundant magnesia, lime, and iron oxide; and into others 
low in silica, some of which are ^unusually low in alumina 
and relatively high in alkalies, and extremely high in mag¬ 
nesia and lime and moderately high in iron oxide, while 
some are high in alumina and alkalies and relatively low in 
magnesia, lime, and iron oxide. 
Thus there is a tendency to concentrate the silica into one 
part of the magma where there is a greater amount of alka¬ 
lies and an average amount of alumina, and to concentrate 
the magnesia, lime, and iron oxide in another part where the 
alumina is about the average and the alkalies are somewhat 
lower. There is a further tendency toward the end of the 
differentiation, which has given rise to the exceptional dike 
rocks and flows, for the portion of the magma low T in silica 
to separate magnesia, lime, and iron oxide from alumina 
and alkalies producing rocks rich in ferromagnesian silicates 
and poor in feldspar, corresponding to certain lamprophyric 
rocks—in this case minettes and leucite-basalts; and also 
producing rocks rich in alkali feldspar and poor in ferro¬ 
magnesian minerals, corresponding to syenite-porphyry, or 
trachyte. 
The exceptional dike rocks and flows of the Absaroka 
range, which will be described in a later part of this paper 
and still more fully in another place, represent the last phase 
of activity of the centers from which the older basaltic rocks 
of this range were erupted. They grade into these basaltic 
rocks and are connected with them in a geological group, 
though for a special purpose they have been considered 
separately. Grouped by themselves, they possess the pecu- 
