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VOLCANIC ROCKS 99 
spots. There is also a porphyritic development containing con- 
. spicuous plagioclase phenocrysts. The spilitic rocks occur princi- 
| pally in the triassic red beds, while the dense types are met with 
in the rhaetic and younger strata. 
The lava rocks have all been disturbed by the tertiary tectonical 
movements, but only exceptionally they are crushed. 
The mineralogical composition of the different types of rocks do 
not reveal any special combination. The melaphyric facies is always 
bound to ophitic structure and richness in ferric oxyde. The ore 
_ impregnation increases incidentally to such a degree as to obscure, 
especially by pronounced weathering, the original mineralogical 
composition and structure. The plagioclase laths, however, are 
often clear and lie thence in an almost opaque mesostasis of the 
ore pigment. 
Description of types. 
In the tilted red sandstone layers of upper Gondwana age along 
the southeastern slope of the Cerro Villa Union already from a 
greater distance there appear black, concordant lava sheets carved 
out by erosion. In handspecimen the rock is of a highly ferriferous, 
spilitic type. The abundance of weathered ferric oxyde conceals 
_ the microscopical structure, except the plagioclase laths. These lie 
isolated in the dark mass, producing a porphyritic structure. Albitic 
twins are scarce, and the extinction is wandering. An incomplete 
decomposition to sericite may be observed almost in all individuals. 
Some of the vesicles are filled with quartz or calcite. 
Another sheet of the same sandstone series represents a dense, 
dark grayish melaphyre. The rock contains abundant plagioclase 
of imperfect lath-form in ophitic arrangement. The albitic twins 
are indistinct. Besides ferric oxyde, partly as secondary product, 
there are no mafic minerals. Among weathering products calcite is 
very abundant. Also small spots of chlorite are visible. The abun- 
dance of the feldspar produces the grayish colour of the rock. 
