52 SIERRA DE UMANGO 
handspecimen the rock shows a marked dynamic flow structure, 
but microscopically it is the most typical of this kind: the parallel 
is produced by the alternate of strokes of quartz grains and feld- 
spar + muscovite grains, the whole showing a great similarity of 
the parallel, crushed granite structures described by Leith.! The 
strokes have been formed by the breaking off of larger grains. The 
quartz bands have afterwards undergone a recrystallisation in some 
degree (see microphotograph, Plate V, fig. 1). The granite described - 
lies in a mica schist complex, strongly pressed too. — In the whole 
mass of the Cerro Villa Union the pressure phenomena are quite 
of the same intensity. 
On the crest of the Sierra de Umango ridge there is, as has been 
mentioned, a zone of red, aplitic granite, concordantly penetrating 
the schists. In handspecimen the granite has a marked parallel 
structure („gneiss-granite“) apparently of dynamical character. Micro- 
scopically the rock shows a middle stage of granulation with many 
variations in size of the grains. The quartz has afterwards 
recrystallized to some extent, as seen from the sinuous contours of 
some larger, unbroken grains. The feldspar is partly albite-twinned 
and very altered by chemical decomposition, partly a badiy deve- 
loped microcline. Mafic minerals are lacking with exception of 
small spots of biotite. 
At the western foot of the same mountain ridge, near to the Ar- 
royo Gaucho, following his border, there is outcropping quite a 
large zone of a red, aplitic granite, bordered upward by the luste- 
ring mica-schist-zone. Because the granite zone runs almost confor- 
mably with a tertiary dislocation line (the Pirca dislocation), this 
rock has undergone an intense crushing. The kataclasis seems to 
increase downward in the mountain slope, being most intense along 
the very fault line (at the foot). The extremely crushed specimen 
shows a habit of an „aplitic schist“ (Plate V, fig. 2). A transversal 
1 Rock cleavage. U. S. Geol. Survey. Bull. 239. 1905. 
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