CRYSTALLINE BASEMENT 33 
on the western slope of the Nevado, shows a medium grained habit, 
| without traces of kataclasis and of a reddish colour. Mafic minerals 
are met with to a very small degree. Microscopically the structure 
seems to be a panallotriomorphic one, with regard to the contours 
_ between the quartz and feldspar constituents. The feldspar is an or- 
thoclase, but generally with a perthitic development showing a kind 
of undulating extinction. This circumstance indicates, that the perthite 
_ probably is of secondary, dynamic origin („dynamic perthite“). Plagio- 
clase is found only in a small degree having a zonar structure with 
transitions between the different zones. Of mafic minerals one finds 
only biotite in small spots and also minute grains of iron ore. The 
rock structure as a whole seems to have quite a slightly altered 
consolidation habit, where dynamic influence is shown only by 
, undulating“ perthite. 
Another handspecimen from the Sierra de Safiogasta, on the road 
from Puerto Alegre to the Safiogasta village, shows a reddish aplitic 
type with quite large grains of somewhat unequal size. The 
microscope (see Plate Il, fig. 1) shows a very distinct inhomogeneous 
structure: small grains mingled with larger ones of allotriomorphic 
habit. The principal constituents are a reddish, pigmented potash 
feldspar and quartz. By greater magnification one finds the feldspar 
- composed of a cryptoperthite. In some parts of this mineral there 
are small rounded inclusions of albitic plagioclase, poicilitically pe- 
netrating the perthitic structure. Also small quartz grains of similar 
- appearance may be observed. These last named ones show a 
common optical orientation over greater spots. The structure of 
the rock has still a fairly primary habit, while only the feldspar 
seems to have been broken up to some extent, but this might also 
be a protoclastic phenomenon, so also the aspect of the quartz mass, 
which is clear and fills up all the sinuous spaces of the feldspar 
individuals. 
A second handspecimen from the Sierra de Safiogasta of a red- 
dish gray biotite granite of coarse, but inhomogeneous grain, con- 
