12 MEMOIRS OF THE CARNEGIE MUSEUM 



silts derived from the eroded material have been laid down in the wide valleys 

 along the courses of parts of the rivers as fluviatile loams, gravels, and sands. 



" The Sandstone 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 sedimentary 

 coverings and the fluvio-marine deposits. The blue-grey rock varies much in depth 

 of color and 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 district in which diabase forms the [p. 23] country are 

 covered up with strata of laterite, frequently over one hundred feet in depth, and 

 in places interspersed with nests of secondary quartz, or traversed by veins and 

 stringers of quartz, or, less often, by lenticular layers of secondary quartz, closely 

 resembling, when cut through by mining shafts, tunnels and trenches, — true quartz- 

 reefs. The quartz rock in all these forms is not unfrequently auriferous, the metal 

 being dispersed through it in a very irregular manner, especially in the large lenticu- 

 lar layers, which in many parts are nearly, or even entirely, barren of gold, and in 

 others are "bonanzas" carrying at rates from twenty to, in places, several hundred 

 ounces of the precious metal to the ton of the rock. Unfortunately hitherto these 

 bonanzas have proved few and far between; but there is no reason for assuming 

 that they will not be found in many places in the enormous area of the laterite 

 deposits which up to the present has not been prospected, as they have been in 

 similar places at intervals in the past. Gold also occurs as paint gold, as gold dust, 

 and as nuggets of very varying sizes in the laterite. 



"Of earlier age than the diabase is the sandstone and conglomerate series. 

 It constitutes the greater portion of the Pacaraima mountains, and spreads west- 

 wardly into Venezuela. A similar formation occurs in Brazil, and in all probability 

 is part of the same massif as the Guiana one. Wherever it occurs it appears to 

 be unfossiliferous, and hence we have no paleontological evidence with regard to 

 the geological period at which it was deposited. Two conjectures have been made 



