40 



Geology of the Gold Fields of British Guiana. 



flornhUndc-granitite-g'ni'isit. Diorite-gneiss. 



Cuyuni 

 River. 



Mazarttni 



River 

 District. 



Essequibo 



River 

 District. 



Mazaruni 



River 

 District. 



Assays were made on large samples of these rocks, using from 

 8 to 16 assay -tons for each determination, and these showed that the 

 granitite-gneiss from the North-Western district yielded gold in minute 

 quantity, amounting to somewhat less than one-tenth of a grain to a 

 ton of the rock, that of the Mazaruni district yielded gold at the 

 rate of 9 grains to the ton, while that of the Essequibo district 

 indicated only minute traces of the metal. The horublende-granitite- 

 gneiss of the Mazaruni district yielded gold upon assay at the rate of 

 1 grain per ton, and that of the Essequibo district had a contents at 

 the rate of 23 grains, whilst the diorite-gneiss of the Mazaruni 

 district yielded at the rate of 8 grains per ton. 



The granitite-gneiss from the Cuyuni was obtained from the district 

 between Tinamu and Payuka Cataracts and an average sample yielded 

 at the rate of 85 grains of silver and -i grains of gold per ton of the 

 rock. Samples taken at the cataracts from near the contacts of the 

 gneiss and the intrusive diabase yielded at the rate of 77 grains of 

 silver and 17 grains of gold per ton of gneiss. 



Hornblende-granitite gneiss from the Mariwa Cataracts in the 

 Cuyuni River 3uelded at the rate of 42 grains of silver and traces only 

 of gold to the ton of the rock. 



Mineral Composition. — The following minerals and aggregates of 

 minerals have been recognised in the specimens of the gneiss which 

 have been examined : — 



1. Original. 



Quartz, orthoclase, microcline, alljite, oligoclase, labradorite, 

 microperthite, muscovite, biotite, hornblende, epidote, augite, ilmenite, 

 magnetite, sphene, garnet, zircon, anatase, spinel and apatite. 



