EPOSTA 
~ = EN 
 VOLCANIC ROCKS 97 
1 going pages, do not belong to this phase, but to earlier and later 
ones. 
As has been stated, the lower Gondwana strata in the Umango 
area are frequently lacking where younger ones exist and where the 
red triassic or even the cretaceous (?) beds lie directly over the 
crystalline basement. This circumstance indicates an erosion hia- 
‚tus, probably caused by the precordilleran (permian) tectonical influ- 
ences. During this time faulting movements seem to have occurred 
in the Umango area like those of the later tertiary time. 
D) VOLCANIC ROCKS OF THE CONTINENTAL SERIES. 
In the pampean sierras-area there are in the continental strata 
many occurrences of lava beds of different composition especially, 
as was stated already by Stelzner (1876—85), in the rhaetic beds. 
These volcanics are nearly all contemporaneous with the intervening 
strata, which is evident from the many amygdaloide rocks and the 
ash beds. — From the Umango area and the surrounding regions 
also many outcrops of similar volcanics are known, principally 
through Bodenbender’s investigations. 
All the continental volcanics in the Umango area are of a very 
basic character. The most common types are melaphyre, porphyrite 
and spilite. Aphanitic varieties do also occur abundantly. The corres- 
ponding tuff layers are partly clayish and very decomposed, partly 
large-grained, or even agglomeratic. 
But little can be stated about the occurrence of volcanic centres 
in the Umango area. The writer has discovered in some parts of 
the crystalline basement crosscutting veins of basic lava rocks, pro- 
-bably corresponding to „furnish or feeding channels“ of the basic 
lava outpourings. At the eastern foot of the Cerro Villa Union there 
is seen a remnant of a volcanic neck, setting up through limestone. 
In the sandstone terranes there are sometimes accumulations of lava 
to a degree as to form doubtless volcanic centres. For instance 
the volcanic mass of the Cerro Rajado at the western entrance of 
