CHAPTER IX. 



THE PETROGRAPHY OF THE CLASTIC ROCKS, SAND- 

 STONES AND CONGLOMERATES. 



The great sandstone and conglomerate formation of the Guianas lies 

 directly upon the older igneous rocks. In places the latter are 

 porphy rites or ijuartz-porphyries, and in others granite or gneiss. In 

 some districts high hills and low mountains rise through the sandstone. 

 These consist of very coarse-grained diabase or of gabbro. The con- 

 glomerates and sandstones appear to have been laid down against the 

 Hanks of these hills. 



The lowest beds of the formation consist of indurated feldspathic 

 raudstones, shales, or grits, as in the Potaro near Amatuk, and near 

 Takwari Mountain on the Essequibo River, or in certain districts — such 

 as the Cuyuni above the mouth of the Akarabisi Creek — of breccias and 

 pudding-stones made up of angular and of rounded pebbles of felsite 

 and quartz-porphyry in a recemented feldspathic matrix. Similar rocks 

 occupy wide areas in the neighbouring Dutch colony of Surinam, 

 chiefly in the Upper Merowyne, and have been described by G. C. du 

 Bois, in his work " Geologischbergmannische Skizzen aus Surinam," on 

 pages 38 and 39, as " Grauwack." 



The basal rocks are grey, reddish-grey or dark-grey, and vary in 

 texture from very fine-grained mudstones to bi-eccias and conglomerates 

 containing pebbles of various sizes up to an inch in diameter. 



Above these is a series of fine-grained mudstones, in places — as near 

 the mouth of the Kuribrong River, in the Potaro River — with obscure 

 to faint indications of stratification, or, as at the foot of Amatuk Fall 

 in the Potaro River, with distinct signs of stratification, and in many 

 places on the Cuyuni River near the boundaiy of the colony and 

 Venezuela, on an island south-east of Takwari Mountain on the 

 Esse(![uibo River, and on the Mazaruni River near the Harimaraka 

 Creek, so finely foliated as to form i^ed and purplish-red shales. In 

 some of the mudstones minute fragments of feldspar, frequently plagio- 

 clase, or in places microcline, of microperthite, and of quartz occur, 

 whilst, occasionally, rounded and more or less angular fragments of felsite 

 are found. Manj^ scattered minute grains of magnetite, of titaniferous 

 iron-ore with some leucoxene, and abundant minute grains of hematite 

 and of limonite occur in them. These clastic constituents are cemented 

 together in parts with a feldspathic cement, through which are 

 developed patches of epidote and films, streaks, and nests of sericite, 

 while in other places they are cemented by secondary quartz. 



