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En VOLCANIC ROCKS 101 
: zone the tectonically downfaulted red sandstone, running along the 
western foot of the Sierra de Umango ridge, contains a concordant 
| sheet of a highly amygdaloide, dark lava rock. All the well rounded 
cavities are filled with calcite. The amygdules, closely packed to- 
gether, lend to the rock in handspecimen an almost oolitic habit. 
_ The rock mass between the amygdules is a very fine grained, ophitic 
_melaphyre, very rich in ore minerals, which form an opaque mass 
between the minute feldspar laths. These are quite altered and 
pigmented by ore dust, and their optical characteristics could not 
be fixed. 
In the Paso de Lamar the series of rhaetic sandstones at the base 
of the profile contains a great number of basic lava intercalations. 
They are, according to Bodenbender (1911), of greenish melaphyres. 
There is also an abundance of the same rock in the rhaetic strata 
in the Cerro Mogote to the south. This lava rock was petrographi- 
cally described by Siepert (1894). The structure is a distinct por- 
phyritic one with isometric phenocrysts of plagioclase, showing partly 
a zonar structure. The groundmass contains small laths of plagio- 
clase, showing flow arrangement. Minute grains of iron oxide are 
scattered over this mass. A pseudomorph of calcite after augite is 
occasionally seen. Serpentine also appears, probably as alteration 
product of the original olivine. Regarding a more detailed mine- 
talogical description I refer to the quoted paper by Siepert. 
In the cretaceous red layers, lying immediately over the rhaetic 
strata in the above named profile, some crosscutting dark eruptive 
veins are seen. In handspecimen this rock is dense but carries 
cavities. Microscopically it reveals a minute ophitic structure, composed 
of plagioclase laths, ore grains and calcite. Also opaque spots of 
ferric oxyde are. abundant, evidently of secondary origin as the cal- 
cite, and visible also in handspecimen. There are no mafic minerals 
_ preserved. The cavities are filled with calcite. Seen between crossed 
nicols the rock is quite dark, probably depending on a not incon- 
siderable amount of glassy substance. The age of these cutting 
