70 Hni (Teolnfjy of the Gold Fields of British Guiana. 



zircon, and apatite — although never present in more than un- 

 important quantities are, as a rule, moi'e abundant in the 

 hornblende-granitites than they are in the granitites. 



(d) Ai((jUe-(/ramtite. — -Some granitites, as, for instance, rocks from 

 Kusawe and from Kartauari Cataracts in the Mazaruni River, 

 contain augite in small quantities, usually in aggregates with 

 brown biotite, but in the Black Creek branch of the Groete 

 Creek in the Lower Essequibo River a rock occurs which is a 

 granitite having augite as the most abundant ferro-magnesian 

 mineral. It is of medium texture and dark-purple in colour. 

 Its specific gravity varies from 2"73 to 2'81. The rock is 

 composed of plates of varying size of orthoclase, the twinning 

 planes of which are frequently bent or curved, plates of 

 oligoclase also of very varying size with the lamellte frequently 

 bent or broken, the relative jsroportions of the two feldspars 

 varying greatly in different parts of the rock ; small patches 

 of microcline are seen in some specimens, whilst micropegmatite 

 is of somewhat rare occurrence, and small irregular patches 

 of quartz occur which show strain-shadows. The ferro- 

 magnesian constituents consist of large patches of very pale, 

 almost colourless, augite, usually with numerous extruded 

 grains of magnetite lying in parallel streaks in the crystals, 

 which in places ai-e changed to green hornblende ; abundant 

 to very abundant plates of dark-brown biotite with well- 

 marked haloes of deeper colour surrounding inclusions of 

 minute crystals of zircon, the laraelkie of the biotite in places 

 curving round masses of augite, and in others being bent or 

 broken ; some plates of original pale-blue hornblende, and 

 a little granular epidote. In some parts of the rock small 

 crystals of zircon are relatively abundant, in others are com- 

 paratively rare, whilst prisms of apatite are occasionally found. 

 Part of the pyroxene of these rocks was originally hypersthene. 

 There is a gradual transition among the granitic rocks from 

 aplite, through granite and granitite, to rocks approximating 

 in composition to syenite and to diorite. 



Basic Secretions in the Granitites — In certain of the masses of the 

 granitites segregation-patches occur which are of much darker colour 

 and of higher specific gravity than the bulk of the rock. The patches 

 are, as a rule, of finer grain and closer texture than the rock from which 

 they have segregated. They consist of plates of oligoclase with inclu- 

 sions of sericite and epidote, some plates of orthoclase, and a few patches 

 of microcline ; irregular areas of quartz, many of which show strain- 

 shadows ; many plates of brown biotite, some of chlorite secondary to 

 biotite, and many of green hornblende, with some granules of epidote. 

 Many crystals and some grains of sphene, with grains and minute 

 crystals of magnetite, and of titaniferous iron ore, and a few small prisms 

 of apatite are present as accessories. These basic secretions approach 

 in composition t(j quartz-mica-diorite. 



