1871.] SAWKINS BEITISH GUIANA. 419 



2, Geolo&ical Observations on British Guiana. 

 By James G. Sawkins, Esq., F.G.S. 



A DETAiLEB geological survey of British Guiana, in the present state of 

 the colony, would be a very difficult and prolonged undertaking, as the 

 only lands not covered by dense forests are the coast alluvium, and 

 the savannas or prairies of the interior ; few roads extend beyond 

 the sugar plantations on the former, and few Indian tracks over the 

 latter ; therefore it must be understood that my observations were 

 chiefly confined to the rivers. 



Along the coast the land is low, at many places (as at Georgetown) 

 below the sea-level, composed entirely of sands, clays, and marine 

 debris brought down by the larger rivers, which have been fre- 

 quently silted up into lakes by the contra-action of the waves of the 

 Atlantic ocean. The extent of this alluvium varies from ten to forty 

 miles inland, where it terminates in low sand-hills of 10-70 feet 

 elevation above the sea-level. These sand-hills extend inland until 

 they are seen resting on granite. They are somewhat rounded by 

 the action of the water of insignificant streams and rain. 



The amount of rain, according to observations by Mr. Sandeman 

 diu"ing ten years, shows an annual average of 102-826 in., the 

 minimum being 68*680, and maximum 133-219 ; these facts will 

 convey an idea of what the denuding influences in this colony are. 



My first excursion was limited to what is called the Pomeroon 

 district, comprising the Wainii, Barama, Barima, and Pomeroon 

 rivers. The first granite was observed on the Wainii and Canyaballi, 

 just below the mouth of the Barama, where it is much weathered 

 and worn into deep furrows, particularly under the rain-droppings of 

 large trees; the crystals of quartz, withstanding the effects of exposure 

 longer than either the felspar or mica, protrude on the surface 

 very remarkably. 



To the south-west of the junction of the Wainii and Barama there 

 is a depression covered vnth alluvium or with lakes, ponds or bayous ; 

 then rise granite, gneiss, and mica-schist, often friable on the surface 

 from the decomposition of iron pyrites ; dykes of greenstone traverse 

 the schistose rocks, and indurate them at their junction. At Dowa- 

 cima falls, on the Barama river, a green mica-schist occurs contain- 

 ing veins of granite. Chlorite-schist also appears in many places, 

 pierced by grey trap dykes. The same series of rocks occurs on 

 the Barima and Wainii ; but those above tide-water are chiefly 

 gneissose, with small veins of quartz, or coarse granite, in which the 

 quartz is in semitransparent crystals an inch in length, and the fel- 

 spar of the same dimensions and of a milk-white colour, while the 

 mica is an inch and a half in diameter and half an inch thick. 



My next excursion was along the western shore of the Essequebo, 

 then up the Cuyani, to the line of Venezuela, then across the land 

 between the Cuyuni and the Puruni, a tributary of the Mazuruni, 

 and then up the Mazuruni towards the highest lands of the colony. 



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