106 | SIERRA DE UMANGO 
Also phenocrysts of this mineral are visible, with an idiomorphic 
development. The augite laths are elongated in directions verti- 
cally to the b-axis, reminding of the limburgite form of pyroxene 
and also of the salite form. The maximal extinction is about 45? 
in sections normally to b. All these augite individuals carry a dar- 
ker border zone. The plagioclase laths are highly decomposed. 
In some parts there are pseudomorphs of fibrous serpentine, pro- 
pably after olivine. Iron ore occurs quite abundantly as spots or 
elongated grains, being partly of secondary origin too. 
In the granite-free zone, consisting mostly of quartzitic schists 
exposed in the Quebrada Salto in the northern part of the eastern 
slope of the Sierra de Umango ridge, there is a concordant dark 
_ eruptive dike. The microscopical structure is of hypabyssal habit, 
but the rock is very decomposed by formation of secondary mine- 
tals (calcite, chlorite, etc.). The principal components are plagioclase 
and a mafic mineral, but the latter only preserved as pseudomorphs. 
The former shows allotriomorphic, the latter idiomorphic forms. 
The tracks of the cleavage in the latter lead to short prismatic 
amphibole. 
In the higher part of the Cerro Cacho on its western slope a 
melanocratic dike cuts across the granite intruded schists. 
The principal minerals of this rock are plagioclase, very altered, 
and a greenish amphibole with slight absorption (actinolite) and 
without pronounced orientation. Quartz is present to a small 
amount, representing the last product of consolidation. Secondary 
minerals are calcite and sericite. Apatite and magnetite are present 
as small individuals. The rock seems to belong to the vogesite — 
odinite series of the lamprophyric group. | 
Basaltic dikes (melaphyres of the continental series?). 
In different localities there are found basaltic dikes cutting across 
the crystalline schists. They are sometimes affected by the tertiary 
movements. Although the rocks lack every relation to the conti- 
