44 The Geology of the Gold Fields of British Guiana. 



feldspar-mosaic, the granules of wliich, thougli apparently clear and 

 unaltered, show a mottled appearance under polarised light. Some 

 zoisite and a little epidote have been developed in the feldspars, and 

 some large elongated granules of sphene of the leucoxene type are 

 present. 



Proterobase occui's in narrow dykes in the Essequibo and 

 Mazaruni districts. The rocks of these vary in specific gravity from 

 2"94 to 3*01. Some of the Essequibo rocks are dark-green in colour, 

 compact in texture, and contain large porphyritic crystals of labradorite 

 which in places attain a length of two inches. These phenocrysts in 

 the rock of a dyke at Dehalabani are usually clear, while those in 

 one at Akenna are white and clouded. The compact portions of the 

 rocks consist of small masses of colourless augite enclosed in pale-blue 

 and green hornlilende, the ragged internal edges of which seem to inter- 

 penetrate their kernels of augite. In places these minerals are replaced 

 by patches of chlorite. Labradorite is present in abundance in small 

 lath-shaped crystals, in places clear, in others more or less clouded, 

 whilst a few small granules of quartz, many irregularly shaped grains 

 of titaniferous iron-ore here and there with leucoxene, and a few 

 crystals of pyrite are present as accessories. The phenocrysts of 

 labradorite are corroded by their matrix, show a zoned structure, and 

 are in places much saussuritised, whilst in others they contain minute 

 inclusions of epidote, sericite and carbonates. 



Other dykes of this rock are not porphyritic, the mass having 

 a structure closely reseml^ling that of the compact parts of the rock at 

 DehalaVjani and Akenna. 



Epidiorlte and Amphibolite. — -The general structures of the 

 epidiorites and amphibolite are well seen in the rocks of the Aruka 

 Hills in the North-Western district, of the Blue Mountain Range near 

 the Cuyuni River, from near Wariri on that river, and of the broad 

 belt of country extending for several miles south of the Turesi Falls on 

 the Mazaruni River, and of the Tiger Creek of the Puruni River. 

 Smaller masses occur in many places, as, for instance, at the Barima 

 Mine, and frequently in relatively narrow dykes in the North- Western 

 district, along the courses of the Cuyuni and the Mazaruni Rivers, 

 and to a less extent in the Essequibo, Potaro, Demerara and Berbice 

 districts. 



The epidiorites and amphibolites from the various districts have 

 many characters in common. Macroscopically they are dark-green, 

 grey or almost black rocks, of specific gravities ranging from 

 2-82 to 3-18. 



The amphibole in them is almost entirely of secondary origin, but 

 occasionally small jilates of original olive-green or brown hornblende are 

 found. The hornblende usually present in them is, in thin sections, 

 either colourless, or of various shades of pale-green, pale-blue, olive- 

 green, green and dark-green. It is usually in large, irregular, ragged- 

 edged plates and aggregates, but in some specimens it is in small plates 

 or grains of the deeper-coloured varieties. In places the masses of 



