1879.] Native Bituimens and the Pitch Lake of Trinidad. 245 
become water-logged, sunk to the bottom and been drawn down 
by the ever-revolving pitch. 
In one case at least within my observation, a recently detached 
portion of one of the islands of vegetation afforded incontestable 
evidence of a horizontal movement of the subjacent pitch to the 
extent of several feet. 
According to the present writer, the true cause of the revolving 
motion of the pitch, and of the structure resulting therefrom, is 
found in a fact pointed out by Wall and Sawkins, but not insisted 
upon or fully appreciated by them, viz: the great diurnal range 
in the temperature of the surface of the pitch. On unclouded 
days the asphalt attains an average temperature of about 140° 
Fahr., and sinks during the night to 70° or 60°, suffering a varia- 
tion of 70° to 80°, which must produce a considerable change of 
volume, especially if we consider the vesicular nature of the 
pitch and the quantity of water which it contains. This expan- 
sion will be superficial, and its chief tendency to extend the 
pitch horizontally. Where the pitch is covered by water it will 
not experience this alteration of volume. The courses of the 
water channels may have been determined originally by slight 
inequalities of the surface, holding shallow sheets of water, or 
drifting sand may have occupied these positions and served to. 
protect the asphalt along these lines from the heat of the sun. 
The main point is that the protected areas would be forced down- 
wards by the expansion of the unprotected areas, and this motion 
once established would continue without interruption until the 
contours of the present surface were developed. 
Nocturnal radiation and consequent contraction could not undo 
the effect of the diurnal expansion, but the equilibrium would 
be and doubtless is maintained by the elevation of pitch from 
below in the center of the areas. The plastic pitch beneath the 
solid crust is sometimes forced upwards through the crevices in 
the bottom of the channels. One interestitig example of this is 
described by Mr. Manross: “ In one of the star-shaped pools of 
water, some five feet deep, a column of pitch had been forced þer- 
pendicularly up from the bottom. On reaching the surface of the 
water it had expanded into a sort of center-table about four feet 
in diameter, but without touching the sides of the pool. The 
stem was about a foot in diameter. I leaped out upon this table 
and found that it not only sustained my weight but the elasticity 
of the stem enabled me to rock it from side to side. Pieces torn 
