Tlia Geology of the Gold Fields of British Guiana. 



Foriiliijfitic- 

 (nv/iti-liorii- 

 hlendc-siienitc. 



Mazaruni 

 River. 



Furphi/ritii'- 

 diorite. 



8am Island, 



Mazaruni 

 River. 



Mira-ijuhbro. 



Mazaruni 

 River. 



(Silica 



Alumina ... 

 Iron peroxide 

 Iron protoxide 

 Magnesium oxide 

 Calcium oxide 

 Sodium oxide 

 Potassium oxide ... 



Water 



Carbonic anhydride 

 Titanium oxide ... 

 Phosphoric anhydride 

 Chlorine ... 

 Iron sulphide ... 

 Manganese oxide... 

 Barium oxide 

 Copper oxide 

 Lead oxide 



50-80 

 9-93 

 3-9,5 

 6-91 

 12-13 

 10-17 

 •2-77 

 1-52 

 0-67 



0-80 



trace 



0-06 



4,5 



018 

 trace 



51 -,50 



10 89 



1-75 



6-84 



13-91 



10-19 



1-18 



0-24 



2-40 



0-20 



trace 



0-11 



trace 



0-65 



trace 



trace 



52-20 

 16-10 

 356 

 5-68 

 6-70 

 8-58 

 2-40 

 0-89 

 0-60 

 0-01 

 2-60 

 0-37 

 trace 

 03 

 0-22 



trace 



100 34 



99-86 



99-94 



With the exception of the apHte and muscovite-granite of the 

 Mazaruni River district, which yielded, upon assaying, at the rate of 

 4 grains of gold per ton of the rock, the aplite of the colony -wdiere not 

 mineralised appears to be free from the precious metals. The granite 

 yielded upon assay traces only of the metal ; while I have not been able 

 to detect any traces of gold in thegranitite of the North- Western, of the 

 Cuyuni, and of the Essequibo-Potaro River districts ; although in bulk 

 samples from the Mazaruni-Puruni district, I found it present at the rate 

 of 5 grains of the metal to the ton of the rock. Only minute traces of gold 

 were detected in the hornblende-granitite of the North- Western and of 

 the Essequiljo-Potaro district, while the hornblende-granitite of the 

 Mazaruni-Puruni district yielded gold when assayed at the rate of 

 2 grains to the ton. The augite-granitite of the Mazaruni yielded gold 

 at the rate of 3 grains, while that of the Groete Creek yielded only 

 traces of the metal. These results are in accordance with the fact that 

 the granitic districts in the colony are seldom, if ever, the sites of 

 payable auriferous deposits. 



The augite-syenite of the Demerara River did not show any trace 

 of gold, while that of the Mazaruni River yielded gold at the rate of 

 6 grains. The porphyritic hornblende-augite-syenite yielded at the rate 

 (tf 10 grains of gold and 54 grains of silver to the ton of the rock. The 

 dioritic rocks usually yield, when assayed, small amounts of gold ; the 

 quartz-mica-diorite from the Mazaruni River yielding at the I'ate of Sgrains 

 of the metal per ton of the rock, whilst the porphyritic-diorite from Sam 

 Island yielded at the rate of 4 grains of gold and 48 grains of silver. 



