CHAPTEE V. 



GENERAL GEOLOGY. 



1 . The Coast-Lands. — The coast-lauds of British Guiana form a 

 plain of marine alluvium, interrupted in a few places by low hills 

 of more or less decomposed country rock, as, for instance, the Maburima 

 and Issorora hills in the North-West, and the hills at Santa Rosa and 

 at Macaseema in the Pomeroon district. The alluvial plain is also 

 traversed by lines of sand-dunes forming low ranges seldom exceeding 

 thirty to forty feet in height in this jxirt of the colony. The sands 

 of these hills consist of white quartz, and the grains are, in the majority 

 of cases, well rounded, showing their wind-blown origin. The alluvial 

 deposits are of considerable but unknown thickness. As they rest upon 

 beds of pipe-clay or impure kaolin it is a matter of great dithculty to 

 decide whether the borings for undei-ground waters, which have from 

 time to time been made in various parts of the coast-lands, have been 

 wholly in the alluvium, or have, as they certainly have done in some 

 cases, penetrated through these beds into the underlying residuary 

 clays. In places, however, the alluvial deposits have been proved for 

 depths of over two hundred feet, and it is possible that in many places 

 their thickness far exceeds this. The cores of the borings show that 

 the alluvial deposits consist of beds of more or less indurated marine 

 muds and sands which have been laid down so as to form beds of clay, 

 of mixed clay and very fine siliceous sand, locally known as " caddy," 

 and of siliceous sands varying much in texture,^ — some beds consisting 

 of sand of extremely fine texture, others of coarser grain, while others 

 again approach in character fine grits or gravels. In places some of 

 the beds contain considerable quantities of decomposing vegetable 

 debris, and these, when drilled into during deep well-sinking operations, 

 in some cases give off inflammable mixtures of gases containing mar>^h 

 gas in considerable quantities. In places the ettusionof the gases has been 

 accompanied by that of small quantities of petroleum, a decomposition- 

 product of the organic matters. The geological age of these beds is 

 uncertain, the lower parts may be of late Tertiary or of Pleistocene age, 

 while the parts now bordering the coast-line are undoubtedly recent. 

 I am inclined to think that their age is, in part at least, similiar to that 

 of the Moruga sands of Trinidad. The sand-beds of these deposits are not 

 unfrequently exposed in the cultivated parts of the coast-land, where 

 they are known as sand-reefs. These form in places oval patches of 

 land raised a few feet above the general level of the surroundinir 



