246 Native Bitumens and the Pitch Lake of Trinidad. (April, 
from the edge of this table sank readily, showing that it had been 
raised by pressure and not by its buoyancy.” 
No soundings have ever been made in this lake and its depth 
is unknown. The thickness of the deposit is of course a factor 
of the first importance in determining whether the supply of 
asphalt from this locality is likely to prove practically inexhausti- 
ble in view of the steadily increasing demand for this material in 
the arts. According to Wall and Sawkins each foot in depth is 
equivalent to 158,400 tons, and they assume the maximum aver- 
age depth at thirty feet, making the total supply 4,752,000 tons. 
Judging by the uniformity of the asphalt and the size of the 
revolving areas, the true mean depth must considerably exceed 
this estimate. It is believed that the pitch could be readily exca- 
vated to a depth of ten or fifteen feet, and from that level iron 
bars could probably be forced to the bottom and the true depth 
accurately ascertained. In considering the question of the prob- 
able permanence of the supply, it is important to remember that 
the material is doubtless still escaping from the underlying 
asphaltic sandstone, though perhaps very slowly. 
As regards its origin, the lake is believed not to differ essentially 
from any of the patches of pitch scattered over the surrounding 
country except in this, that the form of the surface has been more 
favorable for its accumulation. It appears to be simply a large 
puddle of pitch, which has oozed out of the sandstone and col- 
lected in a basin-like depression in that rock. 
The observations of Mr. Wall have placed the vegetable origin 
of this bitumen beyond question. The asphaltic sandrock is rich 
in vegetable remains, and it is possible to trace every step in the 
conversion of these into asphaltum, until the organic texture of 
the wood is entirely obliterated and pure bitumen results, the 
external form of the wood alone remaining. 
The fact that the Island of Trinidad lies between a portion of 
the delta of the Oritioco and the sea, long ago led Sir Charles 
Lyell to adopt the view that the asphalt deposits of Trinidad, in- 
cluding the Pitch Lake, which is on the side of the Island 
towards the delta, represented the drift wood brought down by the 
Orinoco in past geological ages. But I believe he afterwards 
concluded that this explanation, like the wood itself, was rather 
far-fetched. For it can be proved that at the time (Miocene pe- 
riod) when these asphaltic beds were forming, the mouth of the 
_ Orinoco was some one hundred and fifty or two hundred miles 
_ further up stream than at present. 
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