104 SIERRA DE UMANGO 
In the glassy mass there are also fragments of a dull feldspar 
and of calcite. The calcite also fills the cavities in the rock. 
E) CROSSCUTTING ERUPTIVE DIKES IN THE CRYSTALLINE — 
BASEMENT. 
In different localities of the Sierra de Umango crystalline area ; 
there are many dikes mostly of porphyritic rocks and sometimes also 
greater volcanic masses cutting through the crystalline basement 
without visible relation to the covering sediments. Their age 
is therefore in many cases of a somewhat doubtful nature. In a 
few localities the dikes have been affected by the first tertiary mo- 
vements. Generally these rocks are of a petrographical habit diffe- - 
ring from the above described ,continental* lava rocks and show ; 
a more pronounced hypabyssal behaviour. 
Quartzporphyry dikes in Ihe Famatina assimilation zone. 
On the western sides of the rugged crystalline foothills along - 
the western slope of the Nevado de Famatina there appear a num- 
ber of straight running dikes of brown-red colour. The position 
of them is remarkably flat. They show no relation to the old — 
structure. In handspecimen the rock is dense, containing scattered - 
minute phenocrysts of paler colour. Some quartz grains are also 
megascopically visible. In some veins the dense groundmass has — 
a flow structure. The rock is a typical quartzporphyry with a 
granophyric, sphaerolitic groundmass, composed of quartz and a 
red-pigmented feldspar. The phenocrysts are to a greater extent — 
of albitic character and thence give the rock a more keratophyric 
appearance. The quartz phenocrysts have nearly all strongly corro- 
ded contours. No signs of pressure are visible in the rock. 
Lamprophyric dikes. 
In his paper on the geology of the Nevado de Famatina Boden- 
bender (1911) mentions, that ,lamprophyric dikes“ do occur in many 
U 
