General Geology. ^7 



and represent a series of acidic lavas, either surface or deeper-seated, 

 the outflow of which over wide areas of the Guianas characterised 

 a period subsequent to the formation of the basal gneiss ; outflows 

 which the occurrence in places of tuffs and felsitic muds indicates may 

 not have terminated at the commencement of the sandstone and con- 

 glomerate period. Hence chronologically I regard the basal gneisses 

 as older than the porphyries and the schists derived from them ; 

 although in places schists belonging to the porphyry series appear to 

 be intercalated with some of the more highly foliated members of the 

 gneiss. 



Among the gneisses I include the epidiorites and hornblende-schists, 

 the almost massive quartz-diorites, the amphibolites, and the more or 

 less altered diabase-gabbros which are found in intimate relationship 

 with the acidic rocks which make up the mass of the fundamental 

 gneiss. These would appear to represent the basic rocks — probably 

 gabbros, — of the complex which gave rise by its metamorphism to the 

 fundamental gneiss of the Guiana region. 



The intensity of the dynamic metamorphism to which the original 

 rocks were subjected having varied greatly, the degree of schistosity in 

 the gneisses diflers widely, and in sympathy with this so do the eftects 

 of weathering upon them and the character of the country in which 

 thev occur. 



The rocks of this class vary in a gradual, and often in an almost 

 imperceptible manner, from massive, almost granitic, rocks which ofler 

 little evidence of foliation, at times so slight as not to be noticeable in 

 hand specimens, although more or less readily distinguishable in the 

 field where the rocks are seen en masse, through others showing roughly 

 marked apparent beddings, caused by some parallelism in the arrange- 

 ment of their component minerals, to true gneiss showing well-marked 

 foliation. 



The constituents of the true gneiss are arranged in narrow more or 

 less parallel layers, which in places are so much bent, curved and con- 

 torted as to assume the damascened appearance like to that sometimes 

 noticeable on sword-blades or on gun-barrels ; whilst, although as far 

 as my experience goes but rarely, in places the dark ferro-magnesian 

 minerals are in curved folia streaming around unaltered kernels of the 

 acidic minerals. In places the lamin* are very thin, either parallel in 

 their relationship one to another, or very minutely crumpled, and the 

 rocks show the characteristics of crystalline schists. This schistose 

 structure is far more common in the basic layers of the banded varieties 

 of the gneiss than in the acidic ones, although instances of it occur in 

 the latter. It also characterises some of the belts of hornblendic rocks 

 which traverse the gneissose country. 



A study of the gneiss, both in situ and in hand specimens and in 

 thin sections, shows that the gneiss has undoubtedly been derived from 

 granitoid rocks, varying in their nature from aplite through granitite 

 and quartz-diorite to basic rocks probably of a gabbro or diabase-gabbro 

 type, by the action of dynamo-metamorphism, the degree of the 

 mechanical and molecular alterations which they now exhibit being 



