402 
INSTIIATED VOLCANIC SOCKS. 
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, aiid 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, trach3rte, 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 Bio 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 V ene- 
zuela, Guiana, and Brazil, the series of primitive rocks 
were broken by trachj-tes, for these, by their fendillatiou 
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 Cundinamarea, 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 
