F. Dixej/ — ■Lateritiziition in Sierra Leone. 217 



percolating through the centre mass. If at this stage the rocks 

 are exposed to the action of erosive agencies, tlie unaltered sands 

 between the concretions are washed out, and only a coarse red mass 

 remains ; this mass tends to become harder and more porous as 

 time goes on. This process is illustrated at any place where cliffs 

 formed of Pleistocene deposits are being eroded by the sea, for the 

 waves continue to cut out the soft sandy beds under the crust until 

 it falls to the foot of the cliffs in large yellow-brown masses. When 

 newly fallen these masses are rubbly and easily broken ; soon, 

 however, they commence to harden and the soft material between 

 the concretions is worn away, leaving only the red, porous laterite. 

 When the iron is distributed locally and irregularly it forms ferru- 

 ginous strings and pipes traversing a mass of partially compacted 

 sand. 



In the sandy clays the changes take place more slowly, and the 

 growing ferruginous concretions do not harden until a much later 

 stage in the general alteration is reached. Meantime, the partly 

 altered beds remain uncemented and brightly mottled in purple, 

 red, brown, and yellow. As compared with laterites formed from 

 less clayey beds the concretions in the sandy clay are smaller and 

 the resulting mass less coarsely porous. 



The thick beds of white, cream, or grey clays do not alter 

 appreciably, except to acquire irregular films of hasinatite due to 

 the infilling of desiccation cracks. The red and yellow or red and 

 white mottled clays which collect locally in passes and in valleys 

 owe their colouring to the above processes ; they do not contain 

 enough iron to produce hard concretions, and consequently they 

 remain plastic while wet, and become hard and compact when dry. 



(ii) Lateritic Iron-ores. 

 The detrital laterite passes locally into lateritic iron-ore by increase 

 in the percentage of iron oxide. Such iron-ores are well developed 

 along the inner margin of the coastal plain encircling the Colony, 

 and occur principally in the neighbourhood of Devil Hole, near 

 Waterloo. They were derived originally from the magnetite 

 (titanomagnetite), which is such an important constituent of the 

 norite mass forming the Colony. They were accumulated at an 

 earlier period in the same way as black sands are accumulating on the 

 coast to-day. The sea formerly reached as far as the inner margin 

 of the present plain, and as the streams brought down the products 

 of weathering from the hillsides the grains of iron-ore were deposited 

 principally near the shore, and the lighter minerals, chiefly felspar 

 and weathered ferromagnesian crystals, were carried further out, 

 to build lip the plain as now existent. Consequently the beds near 

 the old shore-line contain a higher proportion of iron than those 

 distant from it, and are sufficiently rich locally to rank as iron-ores. 

 The ores have attained their present condition through the oxidation 

 and hydration of a great proportion of the titaniferous magnetite. 



