VARIETIES OE GRANITE. 
381 
The Sierra Parinie is one of tlie most extensive granitie 
strata existing on the globe* * * § ; but the granite, which is seen 
alike bare on the flanks of the mountains and in the plains 
by wliich they are joined, often passes into gneiss. Granite 
is moat commonly found in its granular composition and 
independent formation, near Encaramada, at the strait 
of Baraguan, and in the vicinity of the mission of the 
Esmeralda. It often contains, like the granites of the 
Eocky Mountains (lat. 38° — 40°), the Pyrenees, and Southern 
Tvrol, amphibolic erystals,t disseminated in the mass, but 
without passing to syenite. Those modifications are ob- 
served on the banks of the Orinoco, the Cassupiiare, the 
Atabapo, and the Tuamini. The blocks heaped together, 
which are found in Europe on the ridge of granitic moun- 
tains (the Eiesengebirge in Silesia, the Ochsenkopf in Eran- 
conia), are especially remarkable in the nortli-west part of 
the Sierra Parime, between Caycara, the Encaramada, and 
Uruana, in the cataracts of the Maypures and at the mouth 
of the Eio Vichada. It is doubtful whether these masses, 
which are of cylindrical form, parallelopipedons rounded on 
the edge, or balls of 40 to 50 feet in diameter, are the effect 
of a slow decomposition, or of a violent and instantaneous 
upheaving. The granite of the south-eastern part of Sierra 
Parime sometimes passes to pegmatite,^; composed of lanii- 
nary felspar, enclosed in curved masses of crystalline quartz. 
I saw gneiss only in subordinate layers ;§ but, between 
• To prove the extent of the continuity of this granitic stratum, it will 
suffice to observe that M. Lcchenault de la Tour collected in the bars of 
the river Mana, in French Guiana, the same gneiss-granites (with a little 
amphibole) which 1 observed three hundred leagues more to the west, 
near the confluence of the Orinoco and the Guaviare. 
t I did not observe this mixture of amphibole in the granite of the 
littoral chain of Venezuela, except at the summit of the Silla of Caracas. 
t Schrift-granit It is a simple modification of the composition and 
texture of graniie, and not a subordinate layer. It must not be con- 
founded with the real pegmatite, generally destitute of mica, or with the 
‘ geographic stones’ (piedras mapajas) of the Orinoco, which contain 
streaks of dark green mica irregularly disposed. 
§ The magnetic sands of the rivers that furrow the granitic chain of 
the Encaramada seem to denote the proximity of amphibolic or chloritic 
slate (hornblende or chloritschiefer), either in layers in the granite, or 
superposed on that rock. 
