208 The, (jcology of the Gold Fields of Britis/t Guiann. 



The samples of epidiorite, amphibolite, and liornblende-schists 

 examined all yielded gold at rates ranging from three grains to two 

 penny >v eights per ton. In order to ascertain whether the gold occurs 

 in these rocks only in the veins of quartz which not unfrecjuently 

 traverse them, I obtained from one of the poorest of them a specimen of 

 its iron-ores wliich did ]iot show the presence of free gold, and which T 

 .separated from all (pxartz. On assaying the iron-ores yielded gold at the 

 rate of about li^■e pennyweights to the ton, while the amphibolite from 

 which they had been derived gave at the rate of (n\\y three grains. 



The l)asic rocks alter by weathering into buft-coloured, red, brown 

 or chocolate-coloured, feriuginous, more or less siliceous earths and 

 clays, or laterites. These consist of mixtures in very varying 

 proportions of angular quartz sand and grit derived either directly 

 from the (juartz present in the rock before its degradation, or, 

 secondarily, from the decomposition of its feldspars, pyroxenes and 

 hornblendes ; of kaolinite, arising from the feldspars ; and of limonite 

 or other hydrated oxides of iron, resulting from the decomposition, 

 hydration and oxidation of the various ferro-raagnesian minerals 

 present in the rock. In addition to the decomposition-products the 

 laterite always contains in more or less abundance small grains of 

 ilmenite, grains of magnetite in less quantity, some minute prisms 

 of apatite, a few very minute crystals of zircon, and in places, a few 

 granules of epidote. Usually the secondary quartz occurs in it in 

 angulai' fragments, but in places it forms veins and lenticular 

 .sheets. 



Varying (quantities of the oxides of iron set free during the 

 decomposition of the basic rocks are reduced to or are already in the 

 state of ferrous-iron, become dissolved in percolating waters, and move 

 through the mass of the laterite : parts may exist in solution as fen'ous 

 l)icarbonate, and these, when brought under conditions in wdiich that 

 compound becomes dissociated, are thrown out of solution and oxidised, 

 giving rise to coatings and layers of limonite. Other parts are held in 

 solution by the organic acids of the tropical soil-waters and may be 

 leached out of the laterite, but as the solution undergoes oxitlation 

 with attendant destruction of the acids the oxides of iron are deposited as 

 limonite. The limonite forms either pisolitic grains or more commonly 

 suri'ounds grains of siliceous sand, binding them together where 

 relatively abinidant into ferruginous sandstone, or, w'here they are less 

 abundant, using them as nuclei for the formation of masses of impure 

 concretionary ironstone. Some of these masses are of great size, and 

 form blocks largely exceeding a ton in weight. In places where the 

 concretionary ironstone has been distributed in nodules through the 

 mass of the deposit the argillaceous matter may in time be more or less 

 washed away, the surface becoming covered with layers of ironstone 

 washed out from the mass. This is a very common occurrence in the 

 auriferous districts of Guiana, w-here large areas of the sides and tops 

 of hills composed of basic rocks are covered with an "ironcap." The 

 surface layers and the great blocks of concretionary ironstone generally 

 form ruddy-coloured, cindery-looking masses with -^'ery numerous small 

 cavities, a structure which has caused the Creoles of French Guiana to 



