100 Tlte Geology of the Gold Fields of British Guiana. 



usually in very small proportions, and consist of sphene, magnetite and 

 ilmenite, apatite and zircon, while in those derived from granite, or 

 inuscovite-granite, muscovite is present in some abundance, together 

 with small crystalline grains of corundum. Tourmaline (schorl), topaz, 

 beryl, garnet and spinel are also found in certain residuary deposits 

 derived from metamorphosed granites. The residuary deposits from 

 the basic rocks always contain ilmenite in more or less abundance, 

 magnetite in lesser quantity, some apatite, and, in places where 

 rot^lvs were much metamorphosed prior to detrition, grains of the 

 epidote in small amount. The residuary deposits derived from the 

 unmetainorphosed acidic rocks are at present, at any rate, of very minor 

 economic importance to those derived from the basic ones. They 

 are not, as far as is known, the source of any metal or mineral 

 of economic importance. Small quantities of kaolin have been exported 

 to the United States, and there used in various ways. The following 

 shows the chemical compositions of samples of kaolin from different 

 localities in the colony : — 



Combined water . . . 

 Quartz 



Silica (combined) ... 

 Alumina ... 

 Titanium oxide ... 

 Iron peroxide 

 Calcium oxide 

 Magnesium oxide 

 Potassium oxide 

 Sodium oxide 

 Phospboric anhydride 



Conrantj-ne. 



12-64 

 6-59 

 45-52 

 32-53 

 0-01 

 1-36 

 0-02 

 0-61 

 0-17 

 0-52 



Penal Settlemeut, 

 Mazaruni River. 



99-97 



10-32 

 25-65 

 36-36 

 25-26 

 1-10 

 1-22 



0-11 

 0-48 

 0-25 

 0-01 



99-77 



6-73 



61-74 



23-20 



14-38 



0-60 



2-89 



0-11 

 0-53 

 0-63 

 001 



100-72 



In a few places the acidic rocks are found in a mineralised con- 

 dition, and the residuary deposits from them contain, in addition to 

 some of the minerals already mentioned, gold in dust and small nuggets. 

 Some of the gold found in the Omai Bonanza was doubtless derived 

 from rocks of this sort, which are there highly developed. The 

 mineralised masses in parts have been altered into hydrous micas of 

 the scricite type ; and these now form portions of the residuary 

 deposits, being resistant to weathering agencies and hence not 

 kaolinised. 



The laterite deposits from the basic rocks are of great economic 

 importance. They are the matinx of much of the so-called alluvial gold 

 of the colonv, and, as a rule, they represent the country through which 

 layers and veins of auriferous quartz extend. They consist of red 

 or ochreous, sandy to gravelly clays, with layers of angular quartz 



