94 The Geology of the Gold Fields of British Giciana. 



of tlie constituents of the acidic rock ; and the diabase of the tongues 

 is less basic than is that of the dykes or bosses from which they are 

 apophyses. For instance, the diabase in the neighbourhood of the 

 Great Yukuribi Falls on the Essequibo River has a mean specific gravity 

 of 3-04, while that of a narrow vein intrusive into the gneiss of the falls 

 has one of 2"86 ■ and the diabase of the great mass forming Tiboku 

 Heights in the Mazaruni River has a specific gravity of 3-06, while that 

 of a narrow vein traversing the quartz-granophyre of the Tiboku 

 Cataracts has one of '2'92. 



It is everywhere noticeable that the contact-rocks near the edges 

 of dykes of diabase are far more subject to decomposition than are 

 the masses of the dykes ; and it is frequently a matter of great 

 difficulty to obtain specimens from the actual contacts, the rocks from 

 these, as a rule, in the natural sections at the falls and rapids of the 

 rivers, which are the only ones at our disposal, being more or less com- 

 pletely eroded awa3^ This more rapid degradation of the contact-rocks 

 is doubtless in part due to the higher proportion of pyrite which they 

 usually contain. The gold contained in the placer-gravels in many places 

 has been derived from the degradation of such contact-rocks ; and it 

 has been frec{uently noticed that the metal has been obtained in much 

 larger quantity than has been the rule elsewhere in those parts of claims 

 where the usually greyish-white or blue clay of the bedrock is crossed by 

 bands of red or deep ochre-coloured material, which indicate the former 

 presence of dykes of basic rocks. 



In some places diabase is found which has been much altered — the 

 pyroxenes having been changed into aggregates of epidote and chlorite, 

 whilst the mass of the rock is penetrated by infiltration-products such as 

 the carbonates of iron, magnesium and calcium, and secondary quartz. 

 These are not infrequently accompanied by pyrites — arsenical and in 

 places cupriferous — and by galena. These rocks are in places more or 

 less auriferous, but never to a payable extent. The infiltration-products 

 may have been derived from the decomposition of diabase, which at one 

 time overlaid that from which these residuary masses were derived and 

 which is now represented by great depths of red and ochreous clays 

 and layers of concretionary ironstone. In parts these products are 

 due to mineralisation or impregnation by percolating waters during or 

 after the intrusion of the basic rocks. 



As a general rule the diabase is affected by weathering to only 

 slight depths on its surface, the rock passing in the thickness of a 

 fraction of an inch from an ideally fresh and unaltered condition to 

 that of an ochreous clay ; but in some places, especially in many of the 

 placer-gravels, blocks of diabase which have undergone less well-marked 

 changes are found. The diabase in these has been changed to a 

 greenish, earthy-looking rock, containing much chlorite, viridite, in 

 places epidote, and some pale-blue hornblende ; while the feldspars are 

 more or less kaolinised, secondary quartz being a frequent accessory. 

 The iron-ores, with the exception of pyrite, are but little, if at all, 

 altered ; and, as products of the alteration of the rock, the carbonates 

 of iron, calcium and magnesium are present in abundance. 



