THE ORIGIN OF IGNEOUS ROCKS. 
137 
45.90 to 75.90 per cent, of silica. The location of the diagrams 
laterally in each space indicates whether they belong in the 
more silicious or less silicious part of the series, and is a 
means of comparing their silica proportions with one another. 
All of the diagrams have been placed in juxtaposition in 
order that they may be compared more readily. 
The chemical character of the rocks of the Crandall vol¬ 
cano is shown by the analyses in Table II. Their molecular 
variation is represented by diagram 2; the alumina and 
alkalies by 2a, and the lime, magnesia, and ferrous oxide 
by 2b. 
The analyses of the exceptional dike and flow rocks of the 
Absaroka range are given in Table III. The molecular vari¬ 
ation is shown in diagrams 3, 3 a, and 3 b. The noticeably 
high percentage of water in the analyses of this group is one 
of its characteristics. It is considerably greater in the less 
silicious rocks than in the more silicious. It is largely due 
to the presence of zeolites, probably analcite, which are recog¬ 
nized in some of the less silicious rocks. They appear to re¬ 
sult from the alteration of a feldspathic constituent of the 
groundmass in some cases, the other constituents being 
unaltered, except the olivine, which is more or less serpen- 
tinized. However, it is not certain that all of the water is 
the result of decomposition, for one of the unaltered basaltic 
glasses (analysis. 6, Table II), contains 2.99 per cent., and 
certain allied rocks, described by Lindgren,* from the 
Highwood mountains, Montana, contain abundant hydrous 
minerals, which appear to be of primary origin. 
The analyses of several other groups of rocks from well- 
known European regions were selected to compare with these. 
They were taken from the tables of analyses published by 
Rothf. In doing this care was taken to use only those which 
had been made in recent years and bore evidence of greater 
* Lindgren (W.) Eruptive rocks from Montana. Proc. Cal. Acad. Sci. 
8°. San Francisco, 1890, Ser. 2, vol. 3, pp. 39-57. 
fPoth (J.) Beitrage zur Petrographie der plutonischen Gesteine. 
4°. Berlin, 1873,1879,1884. 
21—Bull. Phil. Soc., Wash., Vol. 12. 
