272 TlMEHRl. 
surface of the land, probably by the a6lion of the waves 
at the date when the sea covered the whole face of the 
country up to the sandstone escarpment. It is true that 
the downward extension of these veins may still exist, 
but covered up as they are by an immense amount of 
drifted matter, the task of unearthing them becomes one 
of much difficulty. 
The general obstacles in the way of exploration 
consequent upon the quantity of detrital matter covering 
the surface of the land and effectively concealing any 
outcrop of strata, can only be appreciated when met 
face to face. Perhaps the most serious drawback 
in prospecting a tropical country is to be found in 
the dense forest which covers the entire surface, com- 
pletely hiding its contour, and rendering it impossible 
to make out its geology except by the painfully slow 
process of examining it step by step in detail. Snow 
might be supposed to be the worst obstacle to geolo- 
gical exploration, but it is by no means so serious a 
disadvantage as dense forest. 
Although not to be exclusively relied upon, the know- 
ledge to be acquired of the geology of a country from 
its general contour and physical features is far from 
being inconsiderable. "The form of organised beings' 1 
remarks Humboldt " varies according to the climate, 
and it is that extreme variety which renders the study 
of the geography of plants and animals so attrac- 
tive ; but rocks more ancient perhaps than the causes 
which have produced the difference of the climate on 
the globe, are the same in both hemispheres. The por- 
phyries" containing vitreous felspar and hornblende, the 
phonolite, the greenstone, the amygdaloids and the ba- 
