424 
GEOLOGY OE 
folded quartz veins (Plate I, b), wliicli vary in size from 
a line to some inclies in diameter, and are folded in a 
most minute and regular manner. 
On an island in the river, near this place, are finely 
stratified crystalline rocks, dipping south from 70° to 
vertical, and sometimes waved and twisted. 
The granite often exhibits a concentric arrangement 
of laminae, particularly in the large dome-shaped masses 
in the bed of the river (Plate II. a, c), or in portions 
protruding from the ground (Plate II. <5). Near Sao 
Gabriel, and in the Uaupes, large masses of pure quartz 
rock occur, and the shining white precipices of the serras 
are owing, I have no doubt, to the same cause. At Pimi- 
chin, near the source of the Rio Negro, the granite con- 
tains numerous fragments of stratified sandstone rock 
imbedded in it (Plate \.a); I did not notice this so dis- 
tinctly at any other locality. 
High up the river Uaupes there is a very curious for- 
mation. All along the river-banks there are irregular 
fragments of rocks, with their interstices filled up with 
a substance that looks exactly like pitch. On exami- 
nation, it is found to be a conglomerate of sand, clay, 
and scoriae, sometimes very hard, but often rotten and 
easily breaking to pieces ; its position immediately sug- 
gests the idea of its having been liquid, for the fragments 
of rocks appear to have sunk in it. 
Coarse volcanic scoriae, with a vitreous surface, are 
found over a very wide area. They occur at Caripe, near 
Para, — above Baiao, in the Tocantins, — at the mouth of 
the l^apajoz, — at Villa Nova, on the Amazon, — above 
Barra, on the Rio Negro, and again up the Uaup&. A 
