i 
‘ig 
- 
VOLCAN IC ROCKS 107 
3 nental sediments, they can probably be regarded as allied with the 
volcanic series of the continental period, and may thence have 
played the part of furnish-channels to the lava sheets of this time. 
The microstructure shows an ophitic development, although signs of 
strong pressure often appear. The plagioclase has the labradorite 
characteristics. There is a pale purplish coloured augite which is 
generally granulated. Secondary minerals are common. Quartz 
- occurs in small quantities, likewise magnetite and pyrite. 
F) TERTIARY (2?) VOLCANIC ROCKS. 
From Tambillos to the north, up to the Umango highland ground, 
the passage goes through a hilly landscape of younger volcanics, 
situated on the southern border of the named upland plain (see 
the figure 15). These volcanics are not associated with the conti- 
nental beds as those in the cases above described (continental lava 
rocks), but form an isolated area, limited by faults from the surro- 
unding crystalline ground. The downwarping of this volcanic mass 
has also preserved it from destruction by the river erosion. As 
these rocks differ in geological and petrographical appearance from 
the continental volcanics, it seems probable that they belong to a 
much later volcanic activity and may be considered as contempora- 
neous with the tertiary andine outpourings of chiefly andesitic rocks. 
The principal rock type of this volcanic series may be classified 
as a feldspar-porphyrite. In handspecimen it is a fine grained rock 
of a dark brown colour, carrying small feldspar phenocrysts with 
lustering cleavage surfaces. Microscopically there is an abundance 
of feldspar, not only as phenocrysts, but also as matrix component, 
where it is mingled with grains of iron ore. The feldspar pheno- 
crysts have idiomorphic contours and develop a core of plagio- 
clase and a shell of potash feldspar. The plagioclase is a sodic 
one. The groundmass feldspar develops minute tabular forms, di- 
vergently oriented. They do not show any albitic twins, but undula- 
