218 
IDDINGS. 
For, if it is necessary, as advocated by the French geolo¬ 
gists, MM. Michel Levy,* de Lapparentf and others, to refer 
the crystallization of certain minerals, as quartz, to the min¬ 
eralizing influence of absorbed vapors-, it is evident that the 
required mineralizing agent is universally present in suffi¬ 
cient quantities, since there are no instances where a magma 
of the requisite chemical composition has failed to crystal¬ 
lize completely with the development of quartz when sub¬ 
jected to the proper physical conditions. 
However, it is probable that differences in the amount or 
in the kind of mineralizing agents produce differences in the 
degree or nature of the crystallization of similar magmas 
which have solidified with the same geological environment. 
It has been suggested by Mr. H. J. Johnston-Lavis j that 
the nature of the rocks surrounding a conduit, through which 
molten magmas pass, materially affects the amount and 
character of the vapors introduced into these magmas, which 
will vary as the surrounding rocks are more or less porous 
and are saturated with different kinds of waters. The effect 
of these vapors on the structure and composition of igneous 
rocks is also discussed by the same writer. 
The effect of differences in the amount of the mineralizer 
in a single magma is well illustrated in the structure of the 
obsidian at Obsidian Cliff, Yellowstone National Park, § 
where the alternating layers of holocrystalline and glassy 
rock appear to be unquestionably due to the irregular dis¬ 
tribution through the -magma of vapors, which in the upper 
portion of the flow have produced alternating layers of 
pumice and compact glass. The mineralizing agent was 
present, however, in the alternate glassy layers as well as in 
the crystallized or in the pumiceous ones, for in the highest 
Structures et Classification des Roches Eruptives.” Paris, 1889. pp. 
5 and 12. 
-j- Revue des questions Scientifiques. Paris, 1888. 36. 
J “ The Relationship of the Structure of Rocks to the Conditions of their 
Formation.’’ Sci. Proc. of the Royal Dublin Soc. Yol. Y (n. s.), part 3, 
July, 1886, pp. 113 to 155. 
§ Obsidian Cliff, Yellowstone National Park, by J. P. Iddings. Seventh 
Annual Report of the Director of the U. S. Geological Survey, Washing¬ 
ton, 1888, p. 287. 
