22 Geology of the Gold Fields of British Guiana. 



alluvium. Observations made in the forest regions since the time of 



Brown's geological reconnaissances of the interior of the colony have 



shown that the residuary deposits cover the country not alone on 



the plains of the lowlands but along the great river valleys and on 



the lower forest-covered parts of many of the hills anfl mountains. 



The heavy, at times torrential, tropical rains have carved out of the 



residuary coverings deep ravines and valleys ; and the gravels, sands and 



silts derived from the eroded material have been laid down in the wider 



valleys and along the courses of parts of the rivers as fluviatile loams, 



gravels and sands. It is in these residuary deposits and in the 



gravels and earths derived from them that the readily available mineral 



wealth of British Guiana is found. 



While a traveller through the heart of the forest-covered regions 



..... ^ 



sees but little beyond this sedentary covering in its varieties as already 



described, and but for the very occasional occurrence of boulders of 

 quartz, diabase, am2:>hibolite, or of quartz-porphyry, and the somewhat 

 rare exposure of the country in ravines and especially at small water- 

 falls, cannot recognise the fact that he is traversing a district the 

 country of which consists of igneous rocks and not of stratified clastic 

 deposits, one journeying by the rivers, especially during the dry seasons 

 of the year, cannot fail to notice at the numerous rapids and low 

 cataracts which interrupt his voyages the rocks underlying the 

 prevailing covering. He recognises that the rock most usually exposed 

 is gneiss, in varieties from massive, almost granitic in texture, to finely 

 laminated, and that in places this is intersected by belts of granite ; and 

 that both are traversed by dykes of basic rocks which often are of 

 diabase. He notices that where these dykes are numerous or extensive 

 the neighbouring districts are traversed by ranges of hills, or the 

 general plain is dotted with hills the surfaces of which are covered with 

 ironstone gravel. And in these observations lies the key to the true 

 history of the soils and earths of the forest region. 



The Sandstones and the Diabase Intrusions. — Large areas of the 

 interior of the colony are occupied by a thick stratified formation of 

 sandstone and conglomerate. Just as the basal igneous rocks are so is 

 this pierced and traversed by dykes of diabase, hence the latter rock 

 must be of later origin than all except the sedentary coverings and the 

 fluvio-marine deposits. The blue-grey rock varies much in depth of 

 colour and in texture, and its varieties will be described in the chapters 

 dealing with the petrography of the colony. 



The diabase intrusions occur in belts, generally stretching across the 

 colony in a north-westerly and south-easterly direction. The intrusions 

 vary from narrow dykes, only exposed in the courses of the rivers during 

 very dry seasons, some being not more than from two to three feet 

 across, to low hills and to mountain ranges, some of which — for example, 

 the Eagle mountains in the Potaro gold district — exceed in height two 

 thousand feet. The tops and sides of the hills and mountains, except 

 where they have suffered great denudation, are covered with ironstone 

 gravel, while the lower parts of the districts in which diabase forms the 



