84 The Geoioyy of the Gold Fields of British Guiana. 



Carbonates are not unfrequently present in some quantity. In the 

 finer-grained members of this part of the formation, which, as before 

 mentioned, resemble shales iu appearance, the microscope reveals only 

 minute granules of secondary quartz and of feldspar, and films of 

 sericite, with very abundant minute specks and grains of limonite and 

 hematite. In some specimens narrow veins of secondary quartz, or 

 occasionally of calcite, are noticeable. 



The lower members of the formation are generally succeeded by 

 fine-grained sandstones. Specimens of these from the neighbourhood 

 of Takwari and of Amatuk consist of grains of quartz with a few 

 fragments of epidote in a feldspathic matrix through which much 

 sericite has been developed. Minute grains of magnetite are scattered 

 in the matrix. 



I have examined a large number of specimens of sandstone from the 

 higher parts of the formation collected from various parts of the colony, 

 and have found a general similarity in them. They are of medium 

 texture, and, where not affected by intrusive dykes of diabase, ai'e of a 

 more or less pronounced red colour. In the immediate vicinity of dykes of 

 diabase the rocks are bleached and indurated to the hardness of quaitzite. 

 They ai'e made up of irregular, more or less rounded, grains of quartz 

 of varying sizes, some few of M'hich show strain-shadows, occasionally a 

 few grains of feldspar and some flakes of muscovite. In the unaltered 

 rocks these constituents are cemented together by limonite and by 

 some argillaceous matter, and in the altei-ed ones by a feldspathic 

 matrix containing much sericite. In some of the specimens small 

 rounded pieces of " felsite " are found, whilst flakes of more or less 

 chloritised biotite, minute fragments of hornblende and of augite, and 

 a few grains of sphene and of zircon have been detected in them. 



The conglomerate beds consist of rounded, more or less flattened 

 pebbles of white (piartz in a matrix of sand-grains cemented together 

 in part by limonite but more usually by a feldspathic or a siliceous 

 material. 



The sandstones from near the mountains of Roraima and Yakontipu 

 and near the course of the Ireng River towards the western boundaiy 

 between the colony and Brazil are feldspathic. They are fine to 

 medium in textui'e, and are made up of small angular and sub-angular 

 grains of quartz and of feldspar, with some larger, more or less rounded 

 ones. The feldspar consists generally of oligoclase, mici'ocline being- 

 present only in subordinate amount. Occasionally a few wisps of 

 muscovite and of biotite, a few granules of epidote, and rarely a 

 minute prism of zircon are found in the zx)cks. Minute grains of 

 iron-ore are commonly present, and where the rocks are not altered 

 their grains are held together by a ferruginous cement. 



Some of these sandstones in the vicinity of intrusions of diabase 

 are converted into quartzites, their grains being cemented or welded 

 together by outgrowths from them of secondary quartz. 



Near the source of the Takatu River micaceous sandstones resem- 

 bling itacolumite are found. They consist of more or less rounded 

 grains of quartz, of plagioqlase, and some of microcline, with numerous 



