10 MEMOIRS OF THE CARNEGIE MUSEUM 



of decomposing vegetable debris, and these, when drilled into during deep well- 

 sinking operations, in some cases give off inflammable mixtures of gases containing 

 marsh gas in considerable quantities. In places the effusion of 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 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, similar 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 surrounding [p. 20] argillaceous 

 soils, and in many others gives rise to long narrow ridges somewhat raised above the 

 general level of the land which they traverse. Their mode of occurrence indicates 

 that they are purely local modifications of the alluvial deposits, — sands separated 

 from the mass of the marine silt by the action of local currents and of the waves, — 

 and thus the sand-beds form more or less lenticular beds occupying, as a rule, no 

 great area. They are, in my opinion, very distinct from the beds of sand which 

 characterize various phases in areas where the land is either rising or falling to any 

 extent in the vicinity of a shallow sea. 



" The general evidence indicates that British Guiana occupies one of the most 

 stable areas of the earth's surface, — one which has been very sloicly rising through long 

 ages* — this slow movement having given rise to the low -rapids which usually 

 mark the termination of the tide-way in the rivers, and possibly which has so 

 altered the contour of parts of the continent on which the colony is situated as to 

 change the main lines of drainage, and thus to make the rivers relatively small 

 streams traversing the deeper parts of the courses and valleys eroded by their 

 predecessors in earlier periods. During a stage in this slow upheaval the low hills 

 already mentioned as occurring in a few places in the alluvial coast-land were in 

 turn rocks and small islands in the shallowing sea which then surrounded them, as 

 now they are surrounded by an apparently unbounded expanse of forest or of 

 marsh. 



"A remarkable feature in parts of the alluvial coast-land is the occurrence of 

 extensive beds of a kind of peat. This is locally known as 'pegass,' and consists 

 of the more or less altered remains of ferns, mosses, and sedges, and of other marsh- 

 loving plants. It resembles in its general character the upper layers of the vegetable 

 matter which are found in peat-bogs in temperate climates. As far as nry observa- 



* The Italics are mine. C. H. Eigenmann. 



