The Petroqraplby of tJiP, Fundamental Gneissose Complex. 39 



raicrocline structure, some microperthite, and with quartz in large 

 irregular patches, in places granulitic, included in the feldspar ; in 

 places bi'oad plates of biotite, scarce small flakes of miiscovite occur, 

 and minute flakes of sericite are found in the feldspar ; or they consist 

 of large irregular plates of microcline with included patches of oligoclase 

 or albite with curved laminte and corroded outlines,and irregular patches 

 of quartz ; the feldspars being frequently cracked and the cracks filled 

 with epidote. The pegmatites ai^e therefore of two kinds, both of which 

 contain irregular areas of quartz, but one consists essentiallyof plagioclase- 

 feldspa'rs with subordinate c[uantities of alkali-feldspar, the other of 

 microcline with subordinate ciuantities of plagioclase-feldspar. The 

 pegmatites frequently change from highly feldspathic veins to veins 

 consisting almost entirely or entirely of quartz. In the feldspathic 

 pegmatites crystals of feldspar are found in places from six to eight 

 inches in length, while the biotite separates out in plates up to two or 

 three inches in diameter. Garnets are of common occurrence in parts 

 of the pegmatite-veins, and fair-sized crystals of beryl and of apatite 

 are found in it in a few localities. 



THE CHEMICAL AND MINERALOGICAL COMPOSITION OF THE GNEISS. 



Chemical Composition. — Analyses have been made of the granitite- 

 gneiss, of the liornblende-granitite-gneiss, and of the diorite-gneiss, using 

 average bulk samples drawn in each case from several localities, with 

 the following results : — 



Silica 



Alumina 



Iron peroxide 



Iron protoxide 



Magnesium oxide ... 



Calcium oxide 



Sodium oxide 



Potassium oxide 



Water 



Carbonic anhydride 



Titanium oxide 



Zirconium oxide 



Phosphoric anhydride 



Chlorine 



Iron sulphide 



Manganese oxide ... 



Barium oxide 



Copper oxide 



North-West- 

 ern District. 



7'2-54 

 16-19 

 1-16 

 1-17 

 0-65 

 3-26 

 4-47 

 0-23 

 0-05 

 nil. 

 0-20 



0-12 



traci'. 

 trace. 



100-03 



GnniiiUc-d lU'iK! 



Cuyuni 

 River. 



67-61 



1-13 

 3-44 

 1-39 

 3-18 

 5-63 

 0-82 

 1-48 

 0-Ofi 

 0-72 

 nil. 

 0-09 

 0-02 

 nil. 

 0-1(1 

 0-11 

 nil. 



100-38 



Mazaruni 



River 

 District 



70-09 

 l.>-12 

 1-72 

 1-13 

 1-22 

 2-(il 

 3-61 

 2-75 

 0-78 



trace. 

 0-67 



trace. 

 0-11 

 0-02 

 0-02 

 0-08 

 nil. 

 (>-04 



09-97 



Essequibo 



River 



District. 



67-40 

 19-06 

 0-71 

 1-31 

 1-90 

 4-30 

 3-16 

 1-52 

 0-30 

 0-32 

 0-34 



0-015 



0-004 

 0-06 



trace. 



100-399 



