The Petrography of the Granitic Rocks. 75 



are moderately coarse-grained ; and are made up of large rounded 

 areas of hypersthene, with others of a pale to almost colourless augite 

 in parts, in part changed to a brownish one, with, in places, smaller 

 masses of olive-green hornblende ; large, irregular, more or less rounded 

 plates of labradorite which here and there intei'penetrate the pyroxene 

 masses, and in places show strain-effects ; and a very few granules of 

 magnetite. 



Mica-uorite. — A fairly coarse-grained, dark-grey rock, which occurs 

 near Hiari Rapids in the Aranamai Creek of the Pomeroon River 

 consists of large plates of labradorite, some showing strain-effects, 

 their lamelloe being bent and parts of them changed into more or less 

 granulated areas of water-clear feldspar ; large irregularly bounded 

 crystals of pyroxene, some of which consist of a nearly colourless augite, 

 whilst most of them are of a pink enstatite or bronzite ; in a few cases 

 the peripheries of the masses of pyroxene are more or less altered to 

 pale-green hornblende with the extrusion of minute grains of magnetite, 

 whilst some of the crystals of enstatite contain inclusions of, or are in 

 parts changed to brown biotite ; numerous aggregates of plates of dark- 

 brown biotite in places, surrounding to various extents the masses of 

 enstatite and some irregular granules of titaniferous iron-ore. 



Elvans in the Granitites. — Some of the masses of granitite are 

 traversed by relatively narrow veins of micro-granite and of felsite. 



The micro-granites vary in colour from light-grey, nearly white, to 

 dark-green, are compact in texture, and have specific gravities ranging 

 from 2'65 to 2'74. They consist of a micro-crystalline to micro-granitic 

 groundmass of feldspar and quartz, with some granular epidote, some 

 chlorite and a little sei'icite ; containing from few to numerous small 

 phenocrysts and a few larger rounded blebs (if quartz showing uniform 

 extinction ; and some large and many small phenocrysts of orthoclase 

 and plagioclase. Where the groundmass is micro-crystalline, the small 

 feldspar-laths generally have corroded outlines. As unimportant 

 accessories the rock contains small laths of biotite, and of chlorite after 

 biotite, a few crystals of apatite and some grains of magnetite, of 

 titaniferous iron-ore and of sphene. 



The felsite consists of small rounded grains of quartz and very small 

 plates of augite in a crypto-crystalline aggregate of feldspar, augite, and 

 epidote, with very minute grains of magnetite. The rock is dark- 

 colourerl, almost black, and under the microscope shows fluxion-structure. 



At Yakiri Creek, on the right bank of the Barima River, where it is 

 crossed by the portage, there is an intrusive dyke in granite. The middle 

 part" is fine-grained and massive, the outer parts, in contact with or 

 close to the granite, being foliated. The schistosity arises from 

 pai'allelism in the arrangement of the biotite and the hornblende 

 present in the rock, and is due to fluxion in the pasty mass during its 

 intrusion into the granitite. The rock is of specific gravitj^ 2 -84, and 

 consists of a very fine-grained micro-crystalline aggregate of feldspar 

 with some quartz ; in which are abundant biotite and hornblende crystals 

 lying in approximateh^ parallel bands with a few small granules of epidote. 



