402 



INSULATED VOLCANIC KOCKS. 



Biliner Stein, in Bohemia, which contains fragments of 

 gneiss embedded in its mass. 



Does there exist in South America another group of 

 rocks, which may be preferably designated by the name of 

 volcanic rocks, and which are as distinct from the chain of 

 the Andes, and advance as far towards the east, as the 

 group that bounds the steppes of Calabozo ? Of this I 

 doubt, at least in that part of the continent situated north 

 of the Amazon. I have often directed attention to the 

 absence of pyroxenic porphyry, trachyte, basalt, and lavas, 

 (I range these formations according to their relative age,) 

 in the whole of America eastward of the Cordilleras. The 

 existence even of trachyte has not yet been verified in the 

 Sierra Nevada de Merida, which * links the Andes the 

 littoral chain of Venezuela. It would seem as if volcanic 

 fire, after the formation of primitive rocks, could not 

 pierce into eastern America. Possibly the scarcity of ar- 

 gentiferous veins observed in those countries may be owing 

 to the absence of more recent volcanic phenomena. M. 

 Eschwege saw at Brazil some layers (veins?) of diorite, 

 but neither trachyte, basalt, dolerite, nor amygdaloid ; and 

 he was therefore much surprised to see, in the vicinity 

 of Eio Janeiro, an insulated mass of phonolite, exactly 

 similar to that of Bohemia, piercing through gneiss. I am 

 inclined to believe that America, on the east of the Andes, 

 would have burning volcanos, if, near the shore of Vene- 

 zuela, Guiana, and Brazil, the series of primitive rocks 

 were broken by trachytes, for these, by their fendillation 

 and open crevices, seem to establish that permanent 

 communication between the surface of the soil and the 

 interior of the globe, which is the indispensible condition 

 of the existence of a volcano. If we direct our course 

 from the coast of Paria, by the gneiss-granite of the Silla of 

 Caracas, the red sandstone of Barquisimeto and Tocuyo, 

 the slaty mountains of the Sierra Nevada de Merida, and 

 the eastern Cordillera of Cundinamarca, to Popayan and 

 Pasto, taking the direction of west-south-west, we find in 

 the vicinity of those towns the first volcanic vents of the 

 Andes still burning, those which are the most northerly 

 of all South America ; and it may be remarked that those 

 craters are found where the Cordilleras begin to present 



