102 



Tha Qpolagy of the Gold Fields of Brtfklt Guiana. 



Much of the iron set free during the decomposition of the basic 

 rocks which is already in the state of, or is i^educed to ferrous iron, is 

 brought into solution by the carbonic acid and the organic acids of 

 percolating waters, and is thus enabled to move through the mass 

 of the laterite. Parts of it exist in solution as ferrous bicarbonate, 

 and these, when brought into conditions where that compound 

 becomes dissociated, are thrown out of solution and oxidised, giving 

 rise to. coatings and layers of limonite. The portion held in solution 

 bj'- the organic acids of the tropical soil waters may be leached out of 

 the laterite but it will be deposited as limonite when the solution 

 undergoes oxidation, with attendant destruction of the organic acids. 

 The limonite forms either pisolitic grains of concretionary ironstone, 

 or more frequently surrounds particles of siliceous sands, binding them 

 together where relatively abundant into ferruginous sandstone, or where 

 the sands are less abundant using them as nuclei for the formation of 

 masses of impure concretionary ironstone. In places these masses 

 of concretionary ironstone form great boulders, some of which 

 largely exceed a ton in weight. 



By the gradual washing away of the finer particles of the laterite, 

 in cases where the concretionary ironstone has been distributed 

 through the deposit, the surface becomes covered with layers of 

 ironstone standing out from the surface of the mass. Where the 

 laterite has been washed from the sides of the hills and rearranged 

 by the action of running-water, either in the ravines or in the 

 stream-valleys, the ironstone, together with more or less quartz-gravel, 

 gives rise to beds of coarse ironstone gi'avels and conglomerates. 



The surface-layers and the great boulders generally are in the 

 form of ruddy, cindery-looking masses with very numerous small 

 cavities, the structure of which caused the Creoles of the neighbouring 

 French colony to name the rock " Roche a Ravet " (cockroach-rock), 

 the small holes serving well for the hiding-place of specimens of Blatta 

 Orientnlis. 



The chemical composition of the concretionary ironstone is shown 

 by the following : — - 



In some specimens of the ironstone the limonite has been changed 

 more or less completely into hematite. 



