VAAlETICb OF GRA.XITE. 8S1 



The Sierra Parime is one of the most extensive granitic 

 strata existing on the globe* ; but the granite, which is seen 

 alike bare on the flanks of the mountains and in the plains 

 by which they are joined, often passes into gneiss. Granite 

 is most 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. Tt often contains, like the granites of the 

 Rocky Mountains (lat. 38 40), the Pyrenees, and Southern 

 Tyrol, amphibolic crystals,t disseminated in the mass, but 

 without passing to syenite. Those modifications are ob- 

 served on the banks of the Orinoco, the Cassiquiare, the 

 Atabapo, and the Tuamini. The blocks heaped together, 

 which are found in Europe on the ridge of granitic moun- 

 tains (the Kiesengebirge in Silesia, the Ochsenkopf in Fran- 

 conia), are especially remarkable in the north-west part of 

 the Sierra Parime, between Caycara, the Encaramada, and 

 tlruana, in the cataracts of the Maypures and at the mouth 

 of the Rio 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 lami- 

 nary felspar, enclosed in curved masses of crystalline quartz. 

 I saw gnoiss only in subordinate layers ; but, between 



To prove the extent of the continuity of this granitic stratum, it will 

 suffice to observe that M. Lechenault de la Tour collected in the bars of 

 the river Maim, in French Guiana, the same gneiss-granites (with a little 

 amphibole) which I observed three hundred leagues more to the west, 

 near the confluence of the Orinoco and the Guaviare. 



f 1 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. 



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 Em-arumada seem to denote the proximity of amphibolic or chloritic 

 slate (hornblende or chloritschiefer), either in layers in the granite, or 

 superposed on that rock. 



