Up the Puruni River. 327 
rocks are present also with their sides showing a sharp 
clean fracture as though quite recently severed from 
their parent rock, and bearing no traces of erosion or 
travel. 
Nothing of the nature of erratic blocks are met with 
in the district. I certainly have seen two immense 
blocks of water-worn white quartz which struck me as 
being of this nature, near the Arapu River, while on the 
way to Roraima in 1884, and I hear there is also one 
such on the summit of Woka or Powis mountain in the 
Cuyuni, near Warire Creek. In the Tiger Creek too, an 
affluent of the Essequebo River, there is a huge obelisk- 
shaped mass of, I think, gneiss, which bears the appear- 
ance of having being conveyed thither by ice. 
How the quartz and other rocks have been shattered 
and broken up can only be to a large extent a matter of 
conjecture. In a great measure the volcanic distur- 
bances have contributed to this, and in addition it is 
necessary to take into consideration the effects of the 
alternations of temperature from the fierce heat of the 
noontide sun to the comparative cool of the evening, 
to which the rocks must have been particularly subject 
before the period when vegetation began to cover the 
whole country. In all our rivers where rocks are found 
in the stream beds and on their sides, huge masses of 
rocks, some of them several hundreds of pounds in weight, 
may be seen broken and riven by the change from heat 
to cold of the surrounding air, some broken in cubical 
blocks whilst others weather in concentric layers. In 
LIVINGSTONE'S " Zambesi," it is mentioned that in 
latitude 12 south and longitude 34 east, the surfaces of 
rock which during the day were heated up to 137 Fahr., 
