Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/569

 2. Geological Observations on British Guiana. By James G. Sawkins, Esq., F.G.S.

A detailed 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 frequently 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 during 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 with 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 Dowacima falls, on the Barama river, a green mica-schist occurs containing 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 felspar 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 Cuyuni, 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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