204 L. V. Da/ton — Geology of Venezuela. 



observations can be linked on witb the fuller and more scientific 

 records of British Guiana, there is evidence that the lowest rocks 

 over the whole area form a complex of ancient igneous or highh' 

 metamorphosed sedimentary deposits, similar in petrological character, 

 both macro- and microscopically, to the oldest rocks of other parts 

 of the globe, and notably to the Lewisian complex of the Scottish 

 Highlands. It seems justifiable therefore to consider them, 

 provisionally at least, as of Archaean or pre-Cambrian age, and as 

 representing some of the most ancient parts of the solid crust of the 

 globe. 



These Archaean rocks occur, as has been stated, over the whole 

 Guayana region, constituting hills in some parts, but generally 

 forming the more or less level platform from which the more notable 

 elevations rise. South of Ciudad Bolivar, where they have been 

 studied in most detail, they include gneisses, hornblende schists, and 

 granites, all containing evidence of great antiquity in geological time. 

 Traversing these are two series of dykes, or intrusions in other form 

 of igneous rocks, one ancient, the other, as will be seen, of later date. 

 The older series consists of acidic material, mainlj' quartz-porphyrites 

 and felsites, which, with the surrounding rocks, have undergone 

 considerable dynamic or regional metamorphism. Traversing tliese, 

 in belts which have a general N.W.-S.E. strike, are dykes and sills 

 of basic material, including basalts and dolerites ; along these 

 intrusions, particularly near their intersection with the older series, 

 the majority of the mineral veins are found. 



The basal complex everywhere consists of the original gneisses, 

 etc., and the acidic intrusives, while, resting upon these at very many 

 points in Guayana, are red and white sandstones and coarse 

 conglomerates, with a few bands of red shale, in a series about 

 2,000 feet thick. All the materials of this series of beds appear 

 to have been derived directly from the gneisses; the strata are 

 ])ractically horizontal and have suffered little fracturing, save where 

 they are traversed by the basic intrusives already referred to, which 

 are thus shown to be of much later date than the acidic rocks. From 

 analogy with Brazil it may be inferred that the basalts, etc., are of 

 Secondary age ; in the sandstone groups the sills frequently expand 

 into laccoliths, and in British Guiana it is found that the protection 

 afforded to the sedimentaries under the intrusions of igneous rock has 

 caused them to remain after the denudation of the surrounding areas, 

 forming peculiar flat-topped mountains with vertical cliffs on every 

 side. Whether all the peculiar elevations of this type, so character- 

 istic of Guayana, are due to igneous intrusions is a point upon which 

 we may one day be enlightened. Certain it is that the Sierra 

 Pacaraima, and the vertical-sided outliers north of it, are composed, 

 for the greater part, of these horizontal sandstones. Mount Roraima 

 gives the best known section of these rocks, and Roraima Series would 

 seem a fitting name for them. 



No fossils are known to occur in the rocks of the Roraima Series, 

 but since they are traversed by Mesozoic intrusives they must be 

 assumed to belong at latest to the early part of that period ; while 

 from their constitutional resemblance to the Torridonian Sandstone 



