F. Dixey — Lateritization in Sierra Leone. 219 



iron-ores containing more than 3 per cent of titanium, there is no 

 immediate prospect of a demand for these ores. 



5. Lateritization as a Factor in Cave Production. 



There are many small caves and subterranean watercourses in 

 the colony, due to the removal of unconsolidated sand from beneath 

 a hard crust of laterite, and streams are known suddenly to appear 

 and disappear, much as they do in limestone districts. Hart's Cave 

 near York, and Devil Hole, near Waterloo, are the best-known 

 instances of these phenomena. 



Hart's Cave opens out on to the foreshore and runs back into 

 the cliff for about 100 yards ; it is about 35 feet in average width, 

 and some 12 feet high, and owes its origin to a stream which runs 

 its whole length and issues at its mouth. The stream has Avashed 

 out the soft sand filling a trough in the norite, which outcrops along 

 the foreshore, and left the hard, lateritic 'crust above standing as 

 a roof. 



The entrance is relatively small, and has been artificially reduced 

 by a stone wall built half-way across it. 



Devil Hole is a swallow-hole a few hundred yards to the north- 

 cast of the flag station of that name. It is a more or less cylindrical 

 pipe in the laterite, 25 feet deep and 6 to 8 feet wide, and it opens 

 out on the coastal plain at a height of nearly 100 feet O.D. Leading 

 away in different directions from the bottom of the pipe are several 

 low passages, which were dry when examined early in the dry season. 



Caves are sometimes formed in the lateritized talus slopes of 

 the steep seaward flanks of the mountains ; the exposed outer 

 layers harden into a crust strong enough to support itself when 

 those below subside or are washed out. When the crust itself 

 collapses wide-open caverns are formed, like those seen on the hill- 

 side above the railway station at Waterloo, and that in the Babadori 

 Valley, below Hill Station. 



6. Summary. 



1. Lateritization has been found to affect a variety of rocks in. 

 Sierra Leone, including norite, granitic rocks, and detrital deposits. 



2. The norite of Sierra Leone is not deeply lateritized except in 

 highly jointed areas or in certain bands readily susceptible to the 

 alteration. It yields a gibbsitic laterite. A core of unaltered 

 norite is enclosed in a light-coloured halo (" zone of leaching " of 

 Professor Lacroix), which passes gradually into a red-brown laterite 

 (" zone of concretion "). The transition from core to laterite is 

 a rapid one. The minerals of the norite are attacked in the following 

 order : felspar, diallage, hypersthene, magnetite (titanomagnetite). 



3. Norite containing only a small quantity of disseminated 

 magnetite is less readily lateritized than norite containing a moderate 

 amount. Where, however, the norite includes an unusually high 

 proportion, as in the streaks of magnetite, lateritization proceeds 

 very slowly. 



