Itevietvs — Harrison s Goldjields of British Guiana. 175 



on the goldfields of the Colony there will be a long future of 

 prosperity before them. 



The gold occurs in two ways — in placers or alluvial deposits and in 

 reefs or mineralized masses of igneous rock. The placers are not 

 true gravels of alluvial origin, but merely the rotted surface accumu- 

 lations covering the gneisses and other igneous rocks which make 

 up the greater part of the country. This material is very thick, 

 and often a considerable overburden covers a foot or two of pay 

 dirt on the bed-rock. It is purely of local derivation, except that 

 the rain-soaked clays creep slowly down slopes after the wet season. 

 The gold is got by sluicing, so that it is a poor man's field, and is 

 mostly very fine, though some large nuggets have been found. The 

 river alluvia also are auriferous, but they have a much more limited 

 distribution. Clearly, if much of the sedentary surface deposits 

 of British Guiana contains gold in payable quantities, this will be 

 an important goldfield, but the dense forests render exploration 

 very difficult. The gold in these alluvia comes from the country 

 rock, and Mr. Harrison is of opinion that it is principally derived 

 from the greenstone or diabase. He has made a very large number 

 of assays of the different types of rock that occur in the goldfields, 

 and has found gold in many of them, but principally in the basic 

 rocks. He holds that by concentration through weathering and 

 solution the gold content of the alluvia has been increased, a propo- 

 sition certainly very reasonable in itself but excessively difficult to 

 prove in the circumstances under which the gold occurs. 



The gold-reefs of the Omai, Barima, and other mines are of a 

 different category. They are associated with pegmatites, alaskites, 

 and albite aplites which recall many well-known auriferous deposits 

 like those of the Alaska Treadwell. Many of them show evidence 

 of pneumatolytic origin. But in the quartz reefs there is a tendency 

 for the gold to occur in quantity only at the junction of diabase with 

 gneiss or granite or other country rock, a fact which, seeing that the 

 basic rock is the latest intruder, suggests again that it is the real ore- 

 bringer. Mr. Harrison states all the available evidence very clearly. 

 He admits the importance of the granites and aplites as fully proved 

 in certain cases ; but he is strongly of opinion that much of the gold 

 came in -with the basic rocks. The impartial caution with which he 

 handles the problems of ore-genesis deserves great praise, and his wide 

 knowledge of the country gives his opinions much weight. 



Since 1890 small diamonds have been found in considerable numbers 

 in various parts of British Guiana. They occur in gravels which 

 may lie 70 or 80 feet above the rivers, but they have never yet 

 been traced to their parent rock. From their distribution it seems 

 probable that most of them have been derived from the sandstone and 

 conglomerate which overlie the gneisses, but this can only be a mediate 

 source. At Omai they were obtained in " a blue clay, a product of 

 the decomposition of gabbro or diabase in situ". Mr. Harrison made 

 experiments on some of the mica-gabbro of Mazaruni and obtained 

 a residue of minute grains, insoluble in acids, which from their 

 optical and other characters he believes may have been diamonds. 

 Unfortunately they were too small and too few to, yield conclusive 



