EIGENMANN: THE FRESHWATER FISHES OF BRITISH GUIANA 11 



tions go it is never as compact as true peat. This is probably due to the deposits 

 of it being seldom more than from two to four feet in thickness. 



"As pointed out by Sir Charles Lyell in his 'Principles of Geology,' a large 

 portion of the sand and clays of the alluvial deposits has been brought by the cur- 

 rents from the mouth of the Amazon River; the burden brought by the present 

 rivers of the colony from the higher districts through which they flow having been, 

 during recent periods, a very subordinate factor in the accumulation of this wide- 

 spread formation; although perhaps in earlier times, before the land had risen to 

 its present level, the river-borne silt may have contributed a large quota to the 

 mass. 



"The Forest Lands and Residuary Deposits. — The alluvial strata extend to 

 depths varying from five to, in places, as much as thirty-five miles from the coast- 

 line and rest upon beds described by C. B. Brown as 'sand and clay deposits.' ' 



[p. 21.] "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 fluvio-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-marine 

 [p. 22] 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 resid- 

 uary 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 

 and mountains. The heavy, at times torrential, tropical rains have carved out 

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



