82 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
Under the microscope the rock is found to consist of isotropic masses 
of glass and kaolinized matter, numerous quartz fragments, broken feld- 
spar crystals, and shreds of ferro-magnesian minerals. 
The chemical composition of this rock is shown by the following 
analysis: 
Silica (SiOg i: cee ees, rae Aealeraes auc 72550 
Iron (Fe,O,) and Alumina (Al,O3)...... 13.95 
Lime (CaQie teat “eG te. series cote ei: I.4I 
Magnesia (MgO)5..yr Be es nie se 
Alkalies (Na.O and KO) e203 +s oer - 1ie7% 
otalin 5. aa se Ni aiee oe 100.93 
The large percentage of alkalies doubtless accounts for the ease with 
which this rock disintegrates in damp foundations and similar places. 
BIOTITE ANDESITE. 
This rock appears to be made up of a light-gray groundmass containing 
small and evenly distributed phenocrysts of triclinic feldspar and varying 
amounts of biotite. It has a rather pleasing mottled appearance and 
is much used as a building stone in foundations. The chief variations 
noted are with reference to the amount and kind of the ferro-magnesian 
constituent. This is predominantly biotite. In some localities, however, 
specimens have been found in which it consists wholly of hornblende. 
Intermediate varieties also occur. Phenocrysts of quartz occasionally 
come in, making transition types toward the rhyolites. These variations 
may be termed dacites. The groundmass shows very slight variation 
in texture in different portions of the mass, but sometimes exhibits green- 
ish and reddish tints. 
The polarization microscope shows the rock to be a rather acid type 
of andesite. Biotite is in small crystals and in most slides not very 
abundant. ‘The plagioclase, as proved by the extinction angles, are rather 
acid labradorites. The groundmass is made up mostly of feldspar in 
approximately equidimensional masses, giving an appearance somewhat 
resembling the well-known granophyric structure of the rhyolites. Quartz, 
however, in most slides, is not present in determinable quantities. 
