610 



857). The amphibolic slate of Angostura (Vol. 

 v, p. 699), and masses of diorite in balls, with 

 concentric layers, near Muitaco (VoL v, p. 691 ), 

 appear to be superposed, not on micaslate, but 

 immediately on gneiss-granite. I could not, 

 however, distinctly ascertain whether a part of 

 this pyritous diorite was not inclosed on the 

 banks of the Oroonoko, as it is at the bottom 

 of the sea near Cabo Blanco (Vol. iii, p. 405), 

 and at the Montana de Avila, in the rock that 

 it covers. Very large veins, with an irregular 

 direction, often assume the aspect of short 

 layers ; and the balls of diorite heaped together 

 in hills, may well, according to the analogy of 

 so many cones of basalt, have issued from the 

 crevices. 



Micaslate, chloritic slate, and the rocks of 

 slaty amphibol, contain magnetic sand in the 

 tropical regions of Venezuela, as in the most 

 northern regions of Europe. The garnets are 

 there almost equally disseminated in the gneiss 

 (Caraccas), the micaslate (peninsula of Araya), 

 the serpentine (Buenavista), the chloritic slate 

 (Cabo Blanco), and the diorite or greenstone 

 (Antimano) : we shall see further on, that these 

 garnets re- appear in the trachytic prophyries 

 that crown the celebrated metalliferous moun- 

 tain of Potosi, and in the black and pyroxenic 

 masses of the small volcano of Yana-Urcu, at 

 the back of Chimborazo. 



