THE ORIGIN OF IGNEOUS ROCKS. 
169 
Exceptional dikes and flows in the Absaroka range .—In 
the region of Crandall basin and along the eastern portion 
of the Absaroka range there are dikes of rocks whose min¬ 
eral composition shows them to be extreme members of the 
series of varieties belonging to several centers of volcanic 
activity. 
They range from rocks rich in olivine and augite, with a 
groundmass of alkali feldspars and ferromagnesian silicates, 
to those with abundant alkali feldspar and mica. Among 
the first kind is one with a groundmass of orthoclase and 
leu cite; another with a less crystalline groundmass of alkali 
feldspar, augite, mica, and magnetite. These grade into 
olivine-bearing augite-minettes, kersantites, camptonites and 
syenite-porphyry or trachyte. They form a series grading 
into the normal basalts of the region and appear from their 
mode of occurrence to be among the last eruptions of the 
centers with which they are connected. 
Their chemical composition, which is given in Table III, 
analyses Nos. 1, 2, 3, 4, 6, 7, 9, 10, 11, 13, 16, 18, 19, and 
diagrams 3, 3a, 3b, shows that they are. the result of the 
chemical differentiation of a phase of the basaltic magma. 
They represent the tendency of this magma to separate in 
magmas rich in magnesia, lime, and iron and low in 
alumina and relatively low in alkalies, and into others low 
in magnesia, lime, and iron and high in alumina and 
alkalies. It is to be remembered that it is characteristic 
of this region that the alkalies are higher for the whole 
group than for those of the Great Basin and some other 
localities. 
By their mineral composition and microstructure, as well 
as by their chemical composition, these exceptional varieties 
of dike rocks exhibit the closest resemblance to the most 
typical of Rosenbusch’s “ ganggesteine,” namely, the lam¬ 
prophyres. They constitute syenitic and dioritic lampro¬ 
phyres and syenite-porphyries, and prove most conclusively 
the correctness of Rosenbusch’s statement that these kinds 
of rocks have resulted from the separation or differentia- 
25—Bull, Phil. Soc., Wa?b., Vol. 12. 
