General Geology. 21 



have been penetrated by the various borings which have been made in 

 the coast districts. I regard the so-called valley-gravels, the sand and 

 clay beds of C. B. Bi'own, as consisting mainly in the districts through 

 which I have travelled, of arenaceous, argillaceous, and laterite beds 

 arising from the decomposition and detrition of granites and gneiss, 

 of quartz-porphyries and porphyrites, and of hoi'nblendic and augitic 

 rocks. They consist of more or less arenacous clays, varying from white 

 and cream-coloured to dark-red or chocolate-coloured, according as they 

 have been derived from acidic, non-ferruginous rocks or from basic, more 

 or less ferruginous ones. These are generally covered by varying thick- 

 nesses of white, cream-coloured, yellow, or ochreous loams and sands, the 

 latter characterising the districts where the leaching and elutriatiou of 

 the decomposed rocks by the slightly acidic soil waters of the tropical 

 forests and by the rain have been more intense. In places the sandy 

 upper layers have been rearranged by the action of flowing waters or 

 of the wind, the sands and gravelly particles have been collected into 

 hollows, or form wind-blown dunes, the layers of wdiich not unfrequently 

 show false bedding. In the neighbourhood of masses of the more basic 

 rocks the gravelly sands are in places cemented together by a liinonitic 

 cement, and give rise to ferruginous sandstones and conglomerates. 

 Where the original rock which has decomposed in situ consisted of 

 gneiss or of schist, with alternating layers of siliceous or feldspathic 

 minerals and of ferro-magnesian ones, the variously tinged and ditierently 

 coloured layers of sands, sandy clays, and more or less ferruginous 

 earthy clays strikingly simulate stratified deposits showing more or less 

 well-marked current-bedding. The laterites formed by the decomposi- 

 tion of the more ferruginous rocks are frequently covered by ironstone 

 gravel, which locally attain a considerable thickness. These residual 

 deposits cover and hide the true country over vast areas of the lower- 

 lying parts of the colony, and form the characteristic sub-soils and soils 

 of our forest regions. 



The parts of the areas covered by these residuary deposits which 

 abut upon the true alluvial beds are in many places traversed by 

 long ranges of sand-dunes, giving rise to low hills which, as in the 

 case of the range traversed by the Demerara-Essequibo Railway, 

 may attain a height of somewhat over two hundred feet. As a rule, 

 their heights do not exceed one hundred or one hundred and twenty 

 feet. The sand of which the upper parts, at any rate, of these dunes 

 (it is possible that in many cases they cover ridges of the residuary 

 deposits) consist is glistening white quartz sand, the grains of which 

 are usually uniform in size over relatively large areas, the majority 

 being well rounded, thus accentuating the wind-blown origin of the 

 dunes. 



C. B. Brown notices that the beds form a low escarpment at the 

 southern limit of the iiuvio-marine deposit, and that this has been 

 taken for a ridge running parallel to the coast. I have not had 

 opportunities of repeating this observation, but accepting its accuracy 

 I consider that the ridge approximately marks the shore-line which 

 existed at the commencement of the deposit of the present fluvio-inarine 



