22 8 Bulletin of Lahoratones of Denison ■ University [Voi. xi, 
and the relations of the intrusive to the surrounding stratified 
rocks may prove a matter of much interest. The latter are 
sandstones usually of a whitish color but, where in contact with 
the igneous rocks, often greatly reddened and altered. The mu- 
tual influence of the two elements is seen on the one hand in 
the production of jasper and other forms of silica, and on the 
other by the hastened decomposition of the igneous rock. This 
volcano is marked on the land office map as “ red sandstone 
hill.” The typical rock of the series is a red trachyte, number 
577. This sample is taken from a large boss near the base but 
is characteristic of the main elevation as well. The hand sample 
is of a purplish to brownish red or reddish grey color with yel- 
lowish flecks and glistening phenocrysts of feldspar. Phases of 
the rock are a deep brick red. The texture is rough, with 
numerous small irregular openings and spongy portions. Evi- 
dence of some disintegration of the feldspars is patent to the 
unaided eye. 
The section is fairly typical of a trachyte without free 
quartz but with feldspars of two or three orders, and a small 
amount of scattered mica. The largest crystals are of relatively 
little altered plagioclase usually occurring in large polysomatic 
groups. These are not numerous but are very striking. The 
measured angles on either side the twinning plane were 14-16 
and 15-17 in the two cases where most satisfactory readings 
were taken, suggesting, in connection with the association, al- 
bite and with this agrees the positive optical behavior. Next 
in order of size are orthoclase crystals which tend to be much 
altered and filled with interpositions. Some of these crystals 
seem to be almost completely altered to a granular material 
but they have not lost the polarization nor twinning phenomena. 
Frequently the vacuolization and interpenetration of the ortho- 
clase has only proceeded to an irregular area in the centre of 
the crystal where the substance is unaltered. In other cases 
the alteration zone is strictly limited to a band near the peri- 
phery of the crystal. Zonary structure, which in some cases 
simulates the plagioclase twinning, is frequent and seems to per- 
tain to the albite. The smaller rods which fill the magma seem 
