74 Tha Geology of tlta Gold Fiekh of British Guiana. 



diabase V)y containiiiij; a good deal of dark -brown mica. It is more or 

 less granitic in structure, and the augite masses are frequently bordered 

 by uralitic hornblende, while the feldspars show strain-effects. 

 Under the microscope it is seen to l>e made up of large, broad 

 laths and i^lates of labradorite, showing in places commencing 

 saussuritisation and in others strain-effects, and with many needle-like 

 prisms of zoisite included in it; large masses of almost colourless augite 

 sliowing in places schiller-structure, and partially converted on their 

 perij^heries into uralitic hornblende, this change being accompanied in 

 places by the extrusion of abundant small grains of magnetite ; many 

 plates of dark-brown biotite ; a few sparse granules of olivine, abundant 

 granules of titaniferous iron-ore and some scattered minute prisms of 

 apatite. 



In order to examine the minerals present in samples of this rock from 

 the Mazaruni River, which are not attacked by acids, about 200 grams 

 of it was coarsely powdered and digested in successive quantities 

 of a mixture of hydrochloric and hydrofluoric acids until all action 

 ceased. The residue was treated with a warm dilute solution of caustic 

 soda, and the undissolved portion was digested with a mixture of 

 hydrofluoric and dilute sulphuric acids. The minute amount of matter 

 which resisted this treatment consisted of minute grains of zircon, and 

 of a few pale-violet, and many neai-ly to quite colourless octahedral 

 crystals. These crystals are all of very high refractive power, the 

 great majority of them consisting of spinel, whilst some are perofskite. 

 A veiy few minute ones resisted the action of boiling strong sulphuric 

 acid, and are of very high refractive power, and thus probably are 

 diamonds. The largest of these is less than one five-hundredth of an 

 inch across. 



Gahhro. — Near the Kuruduni Creek in the Berbice River, near the 

 Cabalebo Creek in the Corentyn River, and in several places on the 

 Ireng, UpjDer Mazaruni, and Cotinga Rivers, somewhat fine-grained 

 gabbro occurs. Its specific gravity varies from 2-95 to 3*05. The rock 

 is a granitic aggregate of masses of nearly colourless augite, frecjuently 

 showing diallagic markings, and in places changed perijiherally into 

 uralitic hornblende, some of the pyroxenes are traversed by cracks, 

 and are more or less altered into serpentine ; of some flakes of brown 

 biotite ; and of large plates of more or less idiomorphic labradorite, which 

 here and there show the effects of strain. In parts the feldspars have 

 ])een changed into areas of confused crystalline aggregates, or are much 

 saussuritised. Small areas of a rather coarse-textured micro-pegmatite 

 of quartz and feldspar, with a few small irregular patches of quartz, are 

 found in it. As subordinate constituents, a few relatively large, 

 irregularly shaped granules of titaniferous iron-ore and of magnetite, 

 some small prisms of apatite, and microlithes of zoisite, epidote and 

 hornblende occur. 



Norite.^At Tamutan Hill, near the Takatu River, and near 

 8irkirtun in the Cai^ucu Mountains, rocks of gabbro-type occur which 

 contain large proportions of hypersthene and are norites. They 



