CHAPTER VI. 



THE PETROGRAPHY OF THE FUNDAMENTAL 

 GNEISSOSE COMPLEX. 



The fundamental gneissose complex of British Guiana resembles to 

 a marked extent the Archean-gneiss of other parts of the world. It is 

 divisible into rocks of two ages — ^the older gneiss usually acidic, but 

 having in places basic beds ahnost inseparably intercalated with it, and 

 the later basic intrusives now represented by an extensive series of 

 epidiorites, amphibolites and hornblende-schists. 



Excellent sections of the gneiss from which characteristic specimens 

 can be obtained are exposed at many of the catai^acts and rapids in 

 the courses of the rivers, especially at those in the lower parts of the 

 Mazaruni River, below and east of Teboco Cataracts. As a rule, 

 however, the country consisting of gneiss is relatively flat and low- 

 lying, whilst the rock is changed to sandy clays for great depths. Where 

 the rivers traverse country of this sort the gneiss is usually only seen 

 when the beds of the rivers are exposed during the extremely dry 

 seasons. The course of the Essequibo River between Gluck Island and 

 Kuratoka Falls is through gneissose country, and exhibits the above 

 characters to a marked extent. 



Tli'i Gu.ri><s. — The prevalence of gneissose rocks over a large area of 

 British Guiana has been alreafly pointed out. The commonest variety 

 of gneiss is a granitite-gneiss with abundant oligoclase-feldsijar, which 

 in parts contain more or less hornblende, and thus jDasses through 

 hornblende-granitite-gneiss to diorite-gneiss. The texture of the gneiss 

 varies very greatly. It is usually somewhat finely foliated, the lamina3 

 in places being highly contorted, whilst in others it possesses a banded 

 structure, and occasionally the more perfectly foliated portions curve 

 round unaltered pieces or eyes of the original rock. It is frequently 

 traversed by veins of very coarsely cr3'-stalline pegmatite, the feldspar 

 crystals of which are generally grey in colour, but are occasionally pink 

 or red, and in places are from one to as much as six inches in length, 

 whilst plates of dark mica (biotite) may attain diameters of from one 

 to one and a half inches. The gneiss is also traversed by veins of fine- 

 grained granite and of aplite, apophyses from the intrusive masses of 

 granite and granitite which occur in places amid it. The rock varies 

 in a gradual and, in parts, almost imperceptible manner from a massive 

 almost granitoidal one, which offers but little evidence of foliation, 

 frequently not noticeable in the hand specimens, although more or less 

 readily distinguishable in the field, to a true gneiss with more or less 



