46 Tha Geology of tlia Gold Fields of British Guiana. 



usually water-clear, but some show the strijB of plagioclase ; in places 

 in the mosaic plates of feldspar larger than its granules are found, 

 these are usually clouded with sericite, and many of them contain 

 needles of colourless actinolite. Small prisms of apatite sparsely 

 occur in the mosaic and in the masses of actinolite. As unimpurtant 

 accessories small grains of sphene of the leucoxene type, prisms 

 of zoisite, granules and crystals of epidote and plates of carbonates 

 are found. 



The hornblende-schists are darker-coloured than the actinolite- 

 schists, and vary in specific gravity from 2-82 to 3-04. They are made 

 up of allotrioinorphic granules of green, olive-green or bluish-green 

 hornblende with others of a usually water-clear plagioclase-feldspar, 

 and with here and there minute grains of magnetite, of titaniferous 

 iron-ore and of pyrite. In some specimens the granules of hornblende 

 and of feldspar are of sensibly similar size, but as a rule the grains of 

 hornblende aj'e much larger than are those which form the granular 

 aggregates of plagioclase-feldspar. Not unfrequently granules of 

 quartz occur in the mosaic, whilst the rocks are traversed by thin 

 films and narrow veins of the same mineral. In places the quartz 

 veins pass into lenticular masses, or gradually become of considerable 

 thickness — from a few inches to as much as four or five feet across — and 

 extend as sheets for long distances through the rock. These sheets, or 

 reefs as they are termed in the colony, are frequently more or less 

 auriferous, whilst in a few j^laces the lenticular masses are auriferous 

 to a marked degree. 



The accessory minerals present in the hornblende-schists — usually 

 iu very unimportant quantities — are apatite, sphene and zircon, whilst 

 chlorite, epidote, zoisite and carbonates occur in very varying 

 proportions and not unfrequently form thin layers and veins in the 

 rock. 



In some places epidote is an important constituent of the mass of 

 the schist ; it is then usually granular, but it is also found in small 

 prisms. These epidote-hornblende-schists are otherwise similar in 

 structure and in composition to the hornblende-schists, of which they 

 are local modifications. 



Another variety occurs in which chlorite is an important con- 

 stituent. A typical example is found near the portage at Arawak 

 Matope Cataracts on the Cuyuni River; it is there a dark-green, 

 well-foliated rock of specific gravity 2-89. It is composed of a very 

 pale-blue hornblende, varying in structure from more or less frayed-out 

 plates to needles in felted masses, the degree of schistosity of the rock 

 being governed by the state of aggregation of the hornblende. In 

 some of the less altered masses of hornblende the structure suggests the 

 presence of small nuclei of augite, but these are seldom recognisable. 

 The ground-mass of the rock consists of a micromosaic of water-clear 

 feldspar with some quartz, grains of epidote, fairly a.bundant patches 

 of pale chlorite and viridite, many needles of actinolite, some minute 

 cubes of magnetite and a few of pyrite. Here and there flakes of 

 chloritised biotite are seen. 



