244 Native Bitumens and the Pitch Lake of Ti vinidad. [ April, 
motion like this going on in a material almost of stony hardness, 
but that such a revolution is constantly taking place over the 
entire surface of this black lake cannot be doubted. The con- 
clusion then to which a close observation leads us in regard to 
the present condition of this singular lake is, not that it has sud- 
denly cooled down from a boiling state as heretofore described, 
but that solid as the material is, it is still boiling, although with 
an indefinitely slow motion. As the descent of the glaciers may 
be considered the slowest instance of flowing in nature, so. the 
revolutions of the scarcely less solid bitumen of this lake may be 
set down as the slowest example of ebullition.” 
Messrs. Wall and Sawkins, on the contrary, deny the existence 
of the revolving motion, and consider that each of the areas rep- 
resents a center of emission where the asphalt has issued from 
the underlying sandstone, “ and gradually advanced until the 
material from the surrounding foci being encountered, further 
progression was impeded, and the accumulation proceeded in the 
_ vertical in place of the horizontal direction.” But the present 
level of the lake has evidently been maintained for ages, and con- 
sequently it is difficult to see why, if this view is correct, the 
asphaltic bosses have not flattened out and closed up the water 
channels. 
Neither of these views can be regarded as entirely satisfactory. 
Mr. Manross ‘is undoubtedly right as regards the circulation, 
though in error as to its cause. He finds unique and conclusive 
evidence of the revolving process in “numerous pieces of wood 
- which being involved in the pitch are constantly coming to the 
surface. They are often several feet in length and five or six 
inches in diameter. On reaching the surface they generally 
assume an upright position, one end being detained in the pitch 
while the other is elevated by the lifting of the middle. They - 
may be seen at frequent intervals all over the lake, standing 
up to the height of*two or even three feet. They look like 
stumps of trees protruding through the pitch, but their parvenu 
character is curiously betrayed by a ragged cap of pitch which 
invariably covers the top and hangs down like hounds’ ears on 
either side.” These fragments of wood are of the same recent 
origin as the leaves and twigs contained in the vesicles of the 
itch. From the surrounding forest or the green islands of the 
lake itself, they have found their. way into the water channels, 
