102 SIERRA DE UMANGO 
veins is perhaps cretaceous, judging from the development of amyg- 4 
dules. 
In the Quebrada Puesto Quemado, on the northern slope of ‘ 
the Cerro Cacho, there appears as intercalation in the strongly F 
downpressed sandstone remnant a dark volcanic sheet, in hand- 
specimen difficult to determine. Microscopically it is of a typical 
mylonitic structure. The principal minerals are plagioclase, crushed 
* into fragments, and ferric oxyde, filling the interstices between the — 
fragments. The latter is probably always of secondary origin. Doubt- 
less, the original rock was a melaphyre, all mafic components of 
which having been transformed into mylonite and altered to a ferri- 
pigmented mass. The habit of the sandstone containing the vol- 
canic layer indicates also, that strong dynamical influences have 
overgone it. 
To the north from Tambillos, the huge masses of volcanic rocks 
are generally of quite a different composition in comparison with 
the above described rock series. It is highly probable, that the 
former belong to a much later volcanic activity than the latter ones, 
and therefore their description may be postponed. However, in the 
vicinity of these volcanic masses there occur in the red sandstone 
beds belonging to the extensive series of the Tambillos valley floor 
some mighty lava beds of amygdaloide character, well visible on 
the path from the Puesto Tambillos up to the Puesto Umango. 
b) „CONTINENTAL“ TUFF-LAYERS. 
In the description of the profile at the western side of the Cerro 
Guandacol is made mention of the occurrence of a mighty tuff 
layer of dense habit and chocolate-brown colour, concordantly in- 
tercalated between the lower and the upper Gondwana series. The 
layer was already observed by Bodenbender (1911) and is indicated 
in the profile on page 69. In handspecimen the tuff rock is com- 
pletely dense and of no conspicuous stratification. The humid rock 
smells clayish. Microscopically the rock seems to be very fine- 
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