Gold in British Guiana. 271 
In the Cuyuni and Puruni districts we find pretty much 
the same series of rocks as occur on the Barima and 
Barama, though it may be remarked that the quartzites 
are less noticeable, while the unstratified rocks rise into 
greater prominence, hinting at a more advanced stage 
of denudation. 
The gold-field of the Upper Massaruni possesses dis- 
tinctive features of its own, and may be described as 
situated on a nearly circular basis of granite which has 
risen up through felsite, and is accompanied on the side 
where the placers are found by eruptions of intrusive 
diorite. 
In the Essequebo and Potaro fields, the latter of which 
includes the Canawarook district, we find a somewhat 
different formation. The country rock is very generally 
granite, or quartz-porphyry through which very extensive 
igneous eruptions have broken. On the Potaro the 
igneous agency is very evident, the porphyry being 
everywhere intersected by extensive dykes and masses 
of diorite. Although fragments of highly auriferous 
quartz are occasionally met with in the alluvium of the 
creeks, it seems probable that the search for quartz lodes 
in this locality is likely to be attended with more 
difficulty than in other parts of the colony. It appears 
to me that the original source of the alluvial gold in the 
Potaro field is to be referred to the order of formation 
known as " conta6l deposits" rather than to area fissure 
veins, but even acknowledging the latter mode of occur- 
rence, there is evidence sufficient to justify the inference 
that the majority of the auriferous veins, together with 
the rocks which enclosed them, have at some remote 
period been violently broken up, and scattered over the 
LL 2 
