Gold jn British Guiana. 269 
beds covers immense tra6ls in many parts of the colony. 
The valleys and ravines with the slopes of the hill-ranges 
over large areas are covered with this deposit, while it 
forms plateaux and hills of considerable extent, and it 
may be interesting to compare this substance with similar 
formations found in other countries. In the gold regions 
of Brazil is found an auriferous superficial deposit of 
broken fragments of ferruginous rocks cemented to- 
gether and called tapanhoa-canga. This canga is com- 
posed of fragments of micaceous iron, specular iron, and 
magnetic oxide of iron, held together by a red, yellow, 
or brown ochreous cement. The cement becomes in 
places so abundant that the embedded pieces are not 
visible, this then forms distin6l deposits of red ironstone. 
It is sometimes richly auriferous in itself, but its chief 
interest consists in its generally overlying the jacu- 
tinga formation, which in Brazil is the most important 
repository of gold. The jacutinga, so called because of 
the resemblance of its colour to the plumage of the 
Brazilian bird Penelope jacutinga , is simply a pulverulent 
variety of itabirite, a rock composed of micaceous specu- 
lar iron-ore, and a little oxide of iron, and manganese, 
with quartz disseminated, and usually carrying free gold. 
It may be noted en passant that the mountains in the 
southern part of British Guiana are composed largely of 
itabirite. The canga of Brazil is not only found in the 
valleys and on the slopes of the mountains, but it covers 
their most elevated ridges and flanks like a sort of 
mantle, three to ten feet in thickness. The deriva- 
tion of this conglomerate has never been satisfacto- 
rily determined by geologists, some have held an 
opinion that it was of plutonic origin, while others, pro- 
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