194 The Gfiolngy of the. Gold FiehU of British Guiana. 



Near Smith's Post Island on the south-eastern edge of the granitite 

 belt at Kumaka, on the north-western side of which Omai is situated, 

 a mineralised aplite or alaskite also occurs. The rock is of specific 

 gravity 2*7, and has the following composition : — 



Smith's Post 

 Island. 



Silica 68-99 



Alumina 18-29 



Iron peroxide ... ... ... ... ... 2-1 



Iron protoxide ... ... ... ... ... 1'62 



Magnesium oxide ... ... ... ... "40 



Calcium oxide ... ... ... ... ... 4-75 



Sodium oxide ... ... ... ... ... 3-15 



Potassium oxide ... ... ... ... '09 



Water -14 



Carbonic anhydride ... ... ... ... 1-10 



Titanium oxide ... ... ... ... trace 



Phosphoric anhydride ... ... ... '004 



Iron sulphide ... ... ... ... ... -004 



Manganese oxide ... ... ... ... Nil. 



Copper oxide ... ... ... ... ... Nil. 



100-638 



The rock is a rather compact greenish one with small white specks 

 of feldspar, and with here and there veins of calcite and others of 

 quartz. It possesses a granitic texture, and consists of cloudy patches 

 of oligoclase with inclusions of epidote and sericite ; irregular patches 

 of quartz, which are generally granulated, the larger ones showing 

 strain-shadows ; scattered patches of epidote with chlorite : and 

 some grains of titaniferous iron with leucoxene ; whilst carbonates are 

 present as alteration-products. 



The carbonates in it consist of 1 per cent, of calcium carbonate and 

 1*26 per cent, of magnesium carbonate. 



The Smith's Post Island mass of alaskite proved to be non- 

 aui-iferous at its outcrop ; but as Lungwitz's observations showed that 

 in the outcrop of the aplite at Omai, at " about ten feet from the tunnel 

 mouth the proportion of gold was nil, something like fifty feet from the 

 starting point of the tunnel it was just demonstrable, and about ninety 

 feet into the interior just weighable ; from now on, the decomposed 

 mass changed gradually into aplite, and the contents of gold in this, as 

 it passed into rock, quickly increased," its barrenness there cannot be 

 accepted as proof that the deeper parts of it may not be auriferous. 



Like the Omai mass the Smith's Post aplite shows no signs of 

 impregnation with micropegmatite. 



Near the Mahdiana Workings of the Inflexible Syndicate, south-east 

 of the present camp, a cut was made through compact felsite into 

 a coarse-textured granophyre, which is intersected by numerous narrow 

 veins of quartz, and contains much iron pyrites. Samples of the 

 granophyre which were assayed yielded at the rate of from two and one- 

 eighth pennyweights to as much as fifteen pennyweights of gold to the 

 ton of rock. 



