Thp, Orujin of thp Vlncer Gold of Gniana. 207 



scale of '' magmatic ((uartz veins," and may liave served as the feeders 

 of g<»ld from tlie magma to the iron-ores of the V)asic rocks. 



Until far more extended and minute examinations of the British 

 Guiana Goldfields have been made than liitherto it is not possible to 

 acce})t Spurr's theory in its entirety. It is closely supported by districts 

 ha\ing structures similar to that of Omai, but not, as far as is known, 

 of those of other districts such as the Potaro, Puruni, and Cuyuni. 



Under the conditions prevalent in the Guianas and in other parts of 

 the tropics, where constant high temperatures, associated with very 

 heavy and frequent rainfalls, cause a rampant growth of vegetation, 

 igneous rocks of all types are subject to relatively rapid decomposition. 

 The insoluble products of this decomposition, wherever shielded by 

 forest-growth from detrition and erosion, remain in situ as a more or 

 less protective cover to the deeper seated rock. Under favcjurable 

 conditions this covering of more or less argillaceous material attains a 

 great depth, for instance of as much as one hundred and fifty to 

 two hundred feet. Many instances of these deep coverings occur in 

 the placer districts of British Guiana. 



Basic rcjcks of the diabase-gabbro type are very subject to chemical 

 decomposition under tropical conditions, and, whilst highly resistent to 

 erosion where protected from atmospheric influence, as in the beds of 

 rivers and streams, wherever exposed are readily attacked. The action, 

 hteing almost purely a chemical one, affects the sheared members of the 

 group, such as the epidiorites and hornblende-schists, in which the 

 numerous planes of foliation allow ready access to the interior of mass, 

 more than it does the massive members — the unaltered gabbros and 

 diabase — which yield only on their bounding surfaces and along their 

 relatively few joint-planes. Hence the gabbro and diaVjase frequently 

 give rise to ranges of hills and to mountains of considerable elevation, 

 while the epidiorites and hornblende-schists are found as comparatively 

 low rolls and domes, or are not distinguishable in the general contour 

 of the country. 



Many examinations of specimens of l)oth classes of the basic rocks 

 were made by Levat, Du Bois, Lungwitz and myself in order to 

 ascertain whether they contained gold. Levat appears, from the 

 wording of his report, to have confined his attention to the amphibolites 

 and hornblende-schist, for which he uses the terms diorite and "grison." 



He reports that the contents of gold in these rocks without being 

 high is very appreciable, the proportions he gives for two samples which 

 did not show free gold being, in round figures, one pennyweight and one 

 and a half peniiy weights of gold per ton, whilst he instances one sample 

 showing free gold which yielded at the rate of fifteen pennyweights of 

 that metal to the ton. Du Bois stated that samples taken from various 

 diabases gave him from three to nine grains of gold per ton. Lungwitz 

 has not, so far as I have been able to ascertain, published any figures 

 showing the contents of gold in the basic rocks he examined, but ho 

 has stated that he has proved its presence in diabase, and in gabbro as well 

 as in other rocks. I found gtjld in the diabase in proportions varying from 

 mere traces to seventeen grains per ton, whilst I found twenty-six grains 

 to the ton in samples taken from diabase in contact with acidic rocks. 



