643 



of Malpasso) which I described above f ; but 

 the irruption of the trachytes across rocks pos- 

 terior to chalk in the Eugan^es, and in other 

 parts of Europe, joined to the phenomenon of 

 the total absence of fragments of pyroxenic 

 porphyry, trachyte, basalt, and phonolithe -f-, 

 in the conglomerats, or fragmentary rocks an- 

 terior to the most recent tertiary soils, renders 

 it probable that the appearance of trapean 

 rocks at the surface of the soil, is the effect of 

 one of the last revolutions of our planet, even 

 where the irruption has taken place by crevices 

 (veins) which cross the gneiss-granite, or tran- 

 sition rocks, not covered by secondary and ter- 

 tiary formations* 



The small volcanic soil of Ortiz, (lat. 9° 28' 

 — 9° 36') forms the antient shore of the vast 

 basin of the Llanos of Venezuela ; it is com- 

 posed on the points where I could examine it, 

 of only two kinds of rocks, namely, of amygda- 

 loide and phonolithe (Vol. iv, p. 281, &c.) The 

 greyish blue amygdaloide contains fendilated 

 crystals of pyroxene and mesotype. It forms 



* Vol. vi, p. 613. 



+ The fragments of these rocks appear only in tufs, or 

 agglomerats, which belong essentially to basaltic soil, or 

 surround the most recent volcanos. Every volcanic forma- 

 tion is enveloped in brechia, which is the effect of the irrup- 

 tion itself. — Leopold von Buch, Resultate geogn, Forsch, 

 p. Mi 



2 u 2 



