108 SIERRA DE UMANGO 
ting extinction. Evidently it is a more acid (trachy-) andesitic | 
rock. io : 
Another rock associated herewith, is a reddish brown porphyrite 
containing pale phenocrysts of feldspar. Microscopically it reveals - 
abundantly potash feldspar as phenocrysts and also as greater frag- | 
ments. The groundmass is composed of a glassy (2) substance 3 
| 
with inhomogeneous pigment of iron ore dust. The glassy residual 
Fig 15. Lava and tuff accumulations (dark hills in the background) of probably tertiary age, seen 
from the eastern wall of the Tambillos valley. Looking north. The dömeshaped summit in the dis- 
tance is the Cerro Cacho. The promontory in the foreground is of the old crystalline basement. 
The even slope is sheet flood gravels. 
mass contains occasionally cavities. There are also smaller frag- 
ments of quartz and the rock may be classified as a quartziferous 
syenitic vitrophyre. 
Intercalated between these lava masses lie tuff layers, gene- 
rally of paler colour and well visible at a greater distance (cf. 
fig. 15). These layers dip all northward. Microscopically, the tuff 
contains abundantly grains of corroded quartz in an allotriomorphic 
mass of pigmented feldspar and small quartz grains, irregularly 
distributed. Another specimen of the tuffs shows a high degree 
