Clastic Rocks, Sandstones and Conglomerates. 85 



long and narrow laths of biotite and of muscovite. Here the felds- 

 pathic sandstones have been practically converted into mica-schist. 



As a general rule this formation may be described as non- 

 metalliferous, the large number of assays which I made of the very 

 extensive series of samples collected from many localities in 1895 by the 

 Kaieteur Conglomerate Expedition showing an average contents of gold 

 of two and a half grains per ton of the rock, the highest yield being 

 at the rate of eight and a half grains, the lowest being nil. As the 

 materials of the sandstones and conglomerates are chiefly quartz 

 and other minerals derived from granites, gneiss, porphyries and 

 porphyrites — rocks which in the colony usually contain only traces of 

 gold — the low contents in this metal of the sandstones and conglomerates 

 are readily accounted for. The quartz-pebbles which lie on the surface of 

 the Kaieteur Savannah are often more or less corroded on their upper 

 exposed surfaces, parts of them having been dissolved by exposure 

 through long ages to the solvent action of rain-water, the portions 

 of quartz between the well-developed crystals having yielded more 

 readily than the crystals themselves, thus leaving the surface roughened 

 with a network of more or less well-defined small crystals. 



In places on the plateau-like summit of Mount Roraima, especially 

 in some of its shallow depressions, the surfaces of the sandstone are 

 covered by coatings of crystalline quartz. The quartz forms hexagonal 

 prisms terminated at one end by pyramids, some of the prisms 

 being from one to two inches in length. It is either an example of a 

 very coarse crystalline quartzite, in which the crystals of secondary 

 quartz are of exceptionally great size, or it is part of a geode of secondary 

 quartz which was formerly surrounded by the sandstone and has since 

 been exposed by denudations. The facts, as far as known to me, favour 

 the former explanation. 



