The Petrography of the Quartz-porphyry, etc. 57 



Tuff>i. — Below Anaripia Itabu, and in the Ttahu neai' Pigeon Island 

 in the Cuyuni River are exposures of a very fine-grained brown- 

 coloured clastic rock, which is finely bedded, and which, near Anaripia, 

 dips at high angles. It consists of a very fine-grained feldspathic dust, 

 with rarely grains of , quartz, the fragments being angular. The finer- 

 textured layers contain minute grains of magnetite in abundance, and 

 alternate with coarser-textured layers, in which the feldspathic frag- 

 ments are plentifully intermingled with limonite. The rock is cemented 

 with sericitic material, and minute cracks are filled with secondarv 

 quartz. Its specific gravity varies from 2"66 to :^'76. 



In the Berbice River below Itabru Gate, and at Little Itabru, 

 reddish-coloured rocks occur, having specific gravities from 2-53 to 2-65, 

 and consisting of a crypto-crystalline to a glassy matrix with sericite 

 developed in it. In this are scattered a few grains of tjuartz, some 

 minute grains of magnetite, and, in places, large numbers of granules of 

 a light-coloured brown to nearly colourless glass. These rocks are 

 traversed in places by narrow veins of quartz. 



In the country between Christianburg and Akyma on the Demerara 

 River, a series of much altered rocks occurs. They are generally red 

 to reddish-grey in colour, and resemble indurated clays. They give 

 rise to the hills at Christianburg and at Akyma Their origin is 

 clearly shown on microscopic examination. They were originally 

 either felsites with fluidal structure or bedded tufis — probably the 

 latter. They are silicitied, and the vesicular hollows in them are 

 lined or filled with tridymite. 



Somewhat similar rocks, but without vesicles, and cemented by 

 quartz, occur in the Potaro River near the junction of the Kuribrong 

 River, and on the Pomeroon River. In the latter place they are 

 associated with quartz and feldspar-porphyry. 



Poryhyroids, Sericite and Chlorite-schists. — It is very usual in the 

 porphyries and the porphy rites for the massive sorts to show a more or 

 less gradual change through i:)orph3a'oids into sericite-schists. This 

 change of massive I'ocks into fissile ones is caused by a development of 

 sericite-mica from the feldspar of the groundmass, usually accompanied 

 by a development of the same mineral in the feldspar-phenocrysts ; 

 of epidote in greater or less abundance, and of very light-coloured 

 actinolitic hornblende, and of chlorite, from the original f erro-magnesian 

 minerals. Where this change has only produced incipient foliation the 

 rocks may be considered as porphyroids, their final stages being either 

 sericitic or chloritic schists, which give little or no indications of 

 their oi'iginal mineralogical composition. 



They fall into three divisions, like the rocks from which they have 

 been produced — sericite-schists containing blebs of quartz derived from 

 the quartz-porphyries, the granophyres, and quartz-porphyrites ; sericite- 

 schists free from blebs of quartz derived from the feldspar-porphyrites 

 or the felsites ; and chlorite-schists derived from the augite and 

 hornblende-porphyrites. Some of them, however, have been derived 

 from granitic rocks, as, for instance, the sericite and chlorite-schists of 

 the jNlariwa Falls on the Cuvuni River. 



