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Ss a Zu ra Te un ln En Bussen ae Zi 
CRYSTALLINE BASEMENT 49 
b) PETROGRAPHICAL CHARACTERISTICS OF THE YOUNGER INTRUSIVES. 
The greater bodies of this class of intrusive rocks in the Sierra 
de Umango area seem to consist of a white-gray syenite, quite 
large grained and very poor in mafic minerals. Only where there 
are inclusions of some basic, strange rocks, an assimilation product 
of a more basic character may be observed. Where these intrusives 
appear in form of dikes, they: are mostly granites and pegma- 
tites, all of acid character. Where crosscutting dikes occur, they 
consist of a finegrained aplite. 
The easternmost occurrences of the white intrusives are met with 
in the crystalline hills of Las Ramaditas, where crosscutting dikes 
of the aplite and also of white pegmatite are visible on quite a large 
scale. The pegmatites are rich in quartz, and have been but slightly 
‚crushed by later movements, thus contrasting with the occurrences 
of the same rock further to the west (except where the rock lies 
between limestone banks). The aplitic dikes are of quite fine even 
grain and show microscopically a somewhat crushed structure, pro- 
bably depending upon a protoclasis, owing to the circumstance, that 
the dikes are cutting straight through the schist mass, without flex- 
ures or ruptures. 
Further to the west, in the abrupt eastern slope of the Cerro 
Villa Union, opposite the hills of Las Ramaditas, there occurs a 
boss of a white syenite. I had no opportunity to study this 
intrusive body and its contact lines. I only collected some speci- 
mens from a river bed leading down the slope from the heart of 
this massive. The rock consists, according to these boulders (huge 
masses), of an almost monomineralic kind of evengrained intrusive, 
an albite-oligoclase mass. The feldspars are generally very altered 
by sericitization. Muscovite is also a constituent, but not abundant. 
Quartz was not detected, neither in handspecimen nor in mic- 
roscopical slides. The structure seems to be completely free from 
crushing phenomena. 
