200 
IDDINGS. 
breccia is quite basaltic looking, while the grayer breccia 
has an andesitic habit. They are very intimately associated, 
however, with no line of demarcation between them. The 
upper breccias are distinguished from the lowest by the 
presence of pyroxene and the absence of biotite. The bot¬ 
tom breccia is made up of hornblende-mica-andesites, the 
upper breccias of pyroxene-andesites and hornblende-py¬ 
roxene-andesites. 
The breccias and flows of massive lava occurring in the 
upper portion of the mountain, differ among themselves in 
color and macroscopical habit, that is, in the abundance and 
proportion of groundmass and phenocrysts. They also differ 
in the relative abundance of the various essential minerals, 
and in the chemical composition of the rocks within certain 
limits. 
They have all the petrographical characteristics of volcanic 
lavas; they are mostly glassy, with a porous or vesicular 
texture; some are compact, and some, holocrystalline. 
The dikes that have broken through these breccias and 
are consequently of younger age, have furnished a series of 
rocks that vary among themselves mineralogically and 
chemically. The earliest of these dikes consist of horn¬ 
blende-andesite with large hornblende phenocrysts; a few are 
pyroxene-andesite. They are so closely allied petrographi- 
cally to the flow-rocks of the uppermost breccia that it is 
probable that they may be the same magmas which solidi¬ 
fied as dikes during the extravasation of the hornblende- 
pyroxene-andesitic breccias and flows. 
Later dikes consist of hornblende-mica-andesite, with 
abundant small phenocrysts of hornblende, biotite and 
plagioclase. Between these two varieties of andesite are all 
possible gradations, caused by the variations in the amount 
of biotite, and in the general habit of the rocks. 
These rocks in turn grade into still more micaceous vari¬ 
eties which carry phenocrysts of quartz, and as the extreme 
variety present a quartz-mica-plagioclase rock with little 
hornblende. These dacites were the last to break up, and the 
