38 SIERRA DE UMANGO 
The principal minerals, are feldspar, very altered, probably 
always plagioclase, and amphibole of ordinary habit. The plagio- — 
clase forms partly porphyroblastic phenocrysts, with extinction angle 
on cuts perpendicular to M and P corresponding to Ab,,An,). The 
contours of the mineral components are generally sinuous. No tracks ~ 
of pressure are visible, and it is possible, that the rock after some 
earlier crushing process has undergone a complete recrystallisation 
with a hornfels structure. Iron ores in minute grains are scattered 
over the whole rock mass. Also biotite is present, showing strong 
absorption and an intimate association with the amphibole. It is 
highly probable, that the biotite is only a metasomatic product of | 
the amphibole, originated by influence of the granitic magma. 
This phenomenon will be treated of at an another occasion later on. 
All stages of transformation of amphibole to biotite are represented, 
and the extremely dissolved breccias show fragments wholly com-. 
posed of biotite rock. 
The last product of the assimilation process of the amphibolite 
by granite is quite a homogeneous rock of true magmatic aspect, a 
syntectic diorite. This rock was already observed by Bodenben- 
der and sometimes called by him ,tonalite‘.? It is surprising 
wide spread in the present rock surface of the foothills. It forms 
the principal rock of the zone. 
This remarkable rock is gray in colour, of large grain and quite 
rough, and its surface weathers in a peculiar way, forming small 
kettles and hollows, like results of a desert deflation. Dark inclu- 
sions of mafic mineral aggregates are common. By its whole ap- 
pearance the rock is well distinguishable from the ordinary Fama- 
tina granite. Contact lines between these two have not yet been 
studied. 
Regarded from a closer distance or as handspecimen the dioritic — 
1 See the Plate III, fig. 1. 
2 According to Bodenbenders handspecimens from the Nevado, kept in the - 
Museum of the Direction of Mines. 
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