328 TlMEHRI. 
cooled so rapidly by radiation at night that, unable to 
sustain the strain of contraction, they split and threw off 
angular fragments from a few ounces to one hundred or 
two hundred pounds in weight. 
These potent forces, assisted by the disintegrating 
influence of the heavy rainfall, have, it is quite possible, 
been able to remove nearly all traces of the great ice 
age which might have been observed in the locality. 
The rainfall in some seasons is certainly stupendous. 
Whilst coming up the river in September 1887, I saw 
from the boat, in some places twenty or twenty-five 
feet over my head, the slender rootlets of pendent bush- 
ropes knotted at that height by boatmen whilst passing 
during the height of the heavy rainy season which pre- 
vailed in that year. 
The section of the soil on the placers varies but little. 
In some, notably that of the Charles Brothers, on the 
Mara Mara, the red earth known as cascajo in Venezuelan 
diggings mav be seen ; but, as a rule, the various forma- 
tions are as follows : — 1. top soil or humus, two to four 
inches; 2. white quartz sand, one to two feet or more ; 
3. quartz gravel, one and a half to three feet, part of which 
may or may not contain gold ; 4. the pay dirt composed of 
heavy iron manganese sand, white clay, and last gold, 
which rests almost invariably on blue clay, probably a 
disintegrated greenstone. This blue clay, it appears, 
has never been penetrated, and although it may in many 
cases be the true bed rock, there seems to be no reason 
why another and lower layer of pay dirt should not 
exist. It might often be found remunerative to sink 
one or more trial holes in search of an understratum ; 
and also to follow the gold deposits up the hill sides in 
