ISO Till', ao.ohuiy of the Cold Fh'hh of nrifis/i Gu.lnia. 



The trend of tlie older l)asic rocks is iiorth-iKtrth-west and south-south- 

 east, whilst the dykes of diabase run approximate!}^ north-east and 

 south-west. East of Arakaka, as at Manikuru, the older rocks have 

 been the main sources of the auriferous deposits. 



The allu^'ial i^old deposits in the valleys in the southern part of the 

 district, from north of the Takutu Creek to Mazawini, on the Barama 

 River, have been derived maitily, if not whijlly, from hornblende-schist 

 and similar rocks. 



The alluvial deposits in the western part of the North- Western district 

 have, as far as they have been examined, for their source hornblende- 

 schist and other rocks closely allied to it. West of the Towakaima 

 Falls, on the Barama River, the gold-bearing rocks are principally 

 epidiorite and its allies. 



Rocks of the same group are sources of gold in the Lower Barama 

 district, liut in the neighbourhoorl of Hoorie Creek sericite-schist is 

 the source. 



Auriferous Quartz Veins. — The epidiorite and the hornblende-schists 

 are traversed in many places by thin veins of quartz, and these, not 

 unfrequently, are more or less auriferous. The basic rocks of the 

 district contain, in addition, small quantities of gold diffused through 

 them. In many places the older basic rocks and the later intrusive 

 diabase have been subject to decomposition to great depth, and where 

 this is the case, in place of small stringers and thin veins of quartz, 

 the reddish ochreous clays resulting from the decomposition contain more 

 or less extensive lenticular masses of quartz, some occurring over 

 sufficiently large areas to be worthy of the term "quartz reef." The 

 decomposition of the ferro-magnesian minerals leaves their oxides of iron 

 either entirely diffused through the mass of the decomposition-products, 

 giving to them their characteristic colour, or in part segregated out into 

 pisolitic grains and masses of concretionary ironstone. The gold 

 originally diff'used through the basic rock has been taken into solution 

 by percolating waters, and precipitated therefrom either on the surfaces 

 of minute specks of the precious metal, or with the silica of the 

 decomposed rock on the thin stringers of quartz which were in the 

 original rock, converting them into more or less extensive lenticular 

 sheets of auriferous quartz. 



The quartz reef at the Barima Mine has lieen traced from the 

 decomposed residua of both basic and acidic rocks into the undecomposed 

 rock, a rather light-coloui-ed actinolite-schist. The country about the 

 Barima, and the neighbouring mines, is an ideal one for the occurrence 

 of auriferous quartz reefs as it consists of epidiorite and hornblende- 

 schist intersected by numerous dykes of diabase. 



(h) The Groete Greeh, Guyuni, Mazaruni and Puruni Rivers 

 Districts. — These extend from the Salt Creek iir the Groete Creek, its 

 north-eastern extremity, to the sources of the Kopang River in the 

 north-west, a distance of about sixty-five miles ; in a south-westerly 

 direction from the Salt Creek to the Issano River, a tributary of the 

 Mazaruni River, a distance of about seventy-five miles ; from the 



