The Petrography of the Fundamental Gneissose Complex. 37 



some of the original biotite remains only partially changed to 

 magnetite and angite. The feldspar crystals for a considemble 

 distance from the contact have been darkened owing to the develop- 

 ments of very minute specks and glassy intrusions, and especially to 

 the production of innumerable exceedingly minute gas-bubbles 

 throughout their mass. 



Diorite-f/neiss. — In many places the granitite-gneiss has intercalated 

 with it bands of a basic gneiss apparently derived from crushed diorite, 

 diabase or gabbro. These are dark-coloured, usually somewhat fine- 

 gi-ained rocks, approaching in general characters to hornblende-schist. 

 Their specitic gravities vary from 2-85 to 3-00, the mean being 2-91. 

 Rocks of this class have been obtained from the Towakaima Falls in 

 the Barama River, from Arimu Creek and from Devil's Hole on the 

 Cuyuni River, from Tupeku, Itaballi Rapids, and the Teboco channel of 

 the Mazaruni River ; from Wolga, Abouia-Malali and Ararapira on 

 the Essequibo River ; and from near the Sandhills on the Demerara 

 River. They are divisible into two classes, one having a granitoidal 

 structure and the other a foliated one. 



The granitoidal rocks are made up of granitic aggregate of large 

 plates of pale blue or of green hornblende with many crystals of brown 

 biotite, the latter in places being secondary to the former ; some plates 

 and abundant granules of epidote ; plates of orthoclase-feldspar 

 clouded with sericite, some of clear microcline, and patches and plates 

 of plagioclase and of a water-clear feldspar ; some small irregular 

 areas of quartz ; some grains of sphene, and a few prismas of zoisite ; 

 whilst small crystals of apatite are present in very varying quantity. 



The foliated varieties have a very close resemblance to hornblende- 

 schists and are composed of allotriomorphic granules, and in places 

 of idiomorphic lath-shaped plates of green hornblende, some of which are 

 poikilitic ; and flakes of greenish biotite usually more or less chloritised, 

 lying in a granular mosaic of water-clear and of plagioclastic feldspars, 

 some granules of which contain minute flakes of sericite and grains 

 of epidote, with some quartz ; a few grains and aggregates of epidote, 

 a few minute prisms of zoisite, and of aj^atite with some zircon, and 

 a few minute grains of magnetite are unimportant accessories. 



Biotite-schist. — In a few places the gneiss contains very abundant 

 flakes of biotite and passes into biotite-schist. Examples of this have 

 been obtained from Matope Cataracts, Arimu Creek, and Devil's Hole 

 in the Cuyuni River ; from Tupeku in the Mazaruni River, and from 

 Tiger Creek in the Puruni River. The rocks are made up of a 

 granulitic mosaic of water-clear and of plagioclastic feldspar and quartz 

 with some larger plates of orthoclase, which is traversed by numerous 

 streams of long and narrow wisps of dark-brown biotite arranged with 

 their long diameters approximately parallel,with some flakes of muscovite 

 and in places with a few small plates of green hornblende and in 

 others with some lath-shaped crystals of epidote and zoisite ; in parts 

 some patches of chlorite occur ; grains of anatase, of garnet, and of 



