Tlhe Petrography of the Fundamental Gneissose Complex. 33 



plates of muscovite are present ; laths of olive-green biotite are iu parts 

 irregularly scattered through the rock, and in others they form streams ; 

 much of the biotite is more or less chloritised, and in places is completely 

 changed into chloi'ite ; in a few specimens sparse granules of pyroxene 

 are present, while in some others pale-green hornblende is found in 

 small quantity: occasionally epidote is present, apparently in original 

 crystals, but more commonly as secondary granules. Grains of sphene, 

 of garnet, of magnetite and of ilmenite, with, in places, leucoxene, 

 minute crystals of apatite, of zircon, and rarely of rutile, form unim- 

 portant accessories. 



3. Granulitic. — Parts of the gneiss where the dynamic forces have 

 been more intense show not only a well-marked foliated structure but 

 the quartz and feldspar are granular. Specimens of this have been 

 obtained from north of Flat Rock in the Barima River ; Takkari Rapids, 

 Wariri, near Quartz Stone, and in the Arimu Creek in the Cuyuni 

 River j Itaballi Rapids and near Teboco in the Mazaruni River; and 

 from Great Saya Falls in the Essequibo. 



These rocks are mainly composed of a confused aggregate of 

 granular and granulitic feldspar and quartz ; most of the feldspar 

 granules are water-clear and not striated, but some show the 

 characteristics of microcline and others those of oligoclase, or in places 

 of albite ; in parts a few large plates of orthoclase are present, and 

 are more or less sericitised. Rarely a few flakes of muscovite are 

 noticeable, greenish biotite is present, either as scattered small scales 

 or in streams, whilst small plates and patches of green hornblende are 

 of rare occurrence. As accessories in unimportant quantities grains of 

 epidote, magnetite, sphene and apatite occur. 



Hornble7ide-g7-anitite-gneisti.— There is usually a well-marked dis- 

 tinction between the granitite-gneiss and the hornblende-granitite-gneiss. 

 Although in the former class some specimens have been found con- 

 taining a few sparsely distributed small plates of green hornblende I 

 have found few that can be considered as on the border between the 

 two classes. The gneiss in the parts of the colony which I have 

 traversed is usually either well-marked granitite-gneiss or equally 

 well-marked hornblende-granitite-gneiss. 



The hornblende-granitite-gneiss is frequently granitoidal in structure 

 and coarse in texture. It is usually of a reddish tinge, but in places 

 has a greenish hue. The biotite and hornblende are generally con- 

 spicuous on freshly fractured surfaces. The specific gravity varies from 

 2-69 to 2-85, but that of by far the greater majority of the specimens 

 collected falls between 2-7-4 and 2-76. The mean specific gravity is 2-75. 



The granitoidal varieties are well seen at Devil's Hole Cataracts in 

 the Cuyuni Ptiver, on the Mazaruni River at Kartauari, near Turesi, 

 and in Teboco Channel ; at Taparoo and at Long Falls on the Puruni 

 River; at Moneri Island and at Moco-Moco on the Essequibo River ; 

 and below the Great Fall or Ororu Marali on the Demerara River. 

 The rocks are made up of granitic aggregates of irregular areas of 



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