28 Geologij of the Gold Fieldfi of British Guiana. 



proportionate to the intensity of the strains and other forces to which 

 they were exposed during their deformation by earth-stresses. 



The commonest variety of the gneiss is a grey or pinkish-grey 

 granitite-gneiss which in plac-es changes into a white or light pink 

 apHte-gneiss, and in others to a darker-coloured hornblende-granitite 

 gneiss, and occasionally to dark-grey or greenish-grey quartz-diorite- 

 gneiss, or to still darker-coloured diorite-gneiss. The grantitite-gneiss 

 in common with the other varieties of gneiss, but perhaps more 

 frequently, is traversed by veins of aplite and of very coarsely 

 crystalline pegmatite. In many places veins of pegmatite by gradual 

 decrease of feldspar and increase of quartz, pass gradually and almost 

 imperceptibly into quartz-veins. As far as I have examined them, 

 quartz-veins of this character do not contain gold in payable quantities. 



In places, usually near intrusions of granite, the gneiss has under- 

 gone marked alteration, the ferro-magnesian minerals being collected 

 together and forming great masses of a basic biotite-gneiss, or more 

 often of hornblende-biotite-gneiss. These highly basic masses alternate 

 with others of aplite-gneiss which fi-equently contain thin veins and 

 small nests of green epidote. In a few places the banded varieties of 

 gneiss contain layers of finely foliated green-coloured epidote-hornblende- 

 schist. 



Although the massive varieties of gneiss pass almost imperceptibly 

 into the more foliated kinds, this is not noticeable at times when the 

 waters in the rivers are high. Then the only rocks seen belonging to 

 this series through long stretches of the rivers appear to be, unless very 

 carefully examined, massive granites ; and this doubtless led Brown 

 and 8awkins to give somewhat undue prominence to the granite rocks 

 of the colony in, at any rate, the more northerly parts of their 

 geological map. 



There is a well-marked difference in the weathering and the degra- 

 dation of the foliated and the more massive varieties of the gneiss. 

 The former yield readily to atmospheric influences, and to the effects 

 of the great differences in their temperature during the day, when in 

 the dry seasons they are exposed to the rays of the sun, and during the 

 night. They either split into small slabs and flat pebbles where the 

 rocks are usually covered by the waters of the rivers, and only in the 

 driest parts of the year are exposed to the sun in the daytime and to 

 rapid radiation of their heat during the nights, or, where they are 

 more constantly exposed to these influences, they undergo degradation to 

 white, grey, or cream-coloured, or ochreous sandy clays ; which, in the 

 parts which are below the usual level of the river, or are otherwise 

 protected from the action of the atmosphere and of that of percolating 

 water, may retain the foliated structure of the original rocks, the 

 positions of the more basic portions being indicated by lines and 

 nests of rusty-looking ochre. The more massive kinds are far more 

 resistant to weathering, and remain as great rounded masses standing 

 out from the surfaces of the foliated varieties, or as rounded rocks 

 where the main mass of gneiss has been degraded into argillaceous 

 products. Where bands of the massive sorts occur the country is 



