26 SIERRA DE UMANGO 
A microscopical slide of the not assimilated rock shows, that the 4 
principal minerals are plagioclase and amphibole. The first named ~ 
is very altered by sericitic growth, and it is impossible to determine © 
the extinction angle on oriented sections. The amphibole has 
a maximal extinction angle of little more than 20° and may con- 
sequently be of the ordinary type. The absorption scheme is the 
usual too. Small grains of poikilitic quartz occur in the amphibole 
individuals. Iron ore is present as small grains thinly scattered © 
through the rock mass. 
The abyssal body of the southern part of the ridge Sierra de 
Umango, called Cerro Cordobés (also Cerro Guecito), contains very - 
abundantly metamorphosed amphibolites. The rocks occur often — 
as isolated rootless masses in the granite body, but interstratifica- — 
tions between sedimentary schists also exist. A handspecimen of 
an amphibolite of the latter kind from the contact with granite shows - 
a kind of porphyritic structure, by small spots of a pale mineral 
contrasting with the dense dark mass. U. M. the spots or „pheno- 
crysts“ consist of aggregates of a very dull feldspar. The dark mass 
is composed chiefly of a green pleochroic amphibole and also of 
minute feldspar grains. Grains of iron ore are abundantly dispersed 
throughout the rock mass. The feldspar aggregates often contain 
garnet individuals of reddish colour in their centre, indicating that 
a recrystallisation of the rock has taken place. 
An amphibolitic type of a somewhat different composition occurs 
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as an isolated sheet in a „promontory“ or rock cape on the way © 
between Maz and Villa Union, i. e. on the marked northeastern | 
corner of the cerro Villa Union. The rock is characterized by a bril- - 
liant lustre and an almost black colour on the surface. It forms 
a vertical sheet holding red granite on both sides. Already in hand- - 
specimen the rock shows a monomineralic composition, an impres- — 
sion confirmed by the microscopical investigation. The mineral is | 
a dark green, intensely lustering one of the amphibole group. Schisto- - 
sity is not developed in the rock, and the whole structure lacks | 
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