524 Correspondence — F. Dixey. 



1. Quartz-\yolfram lodes. Little cassiterite, sulphides, or tourma- 

 line, characteristically no chlorite. Upper level of tin and tungsten 

 zone. Occurring in the slate or just in the granite, at or near the 

 crest of the mass. 



2. Quartz-chlorite-wolfram-cassiterite-mispickel lodes. Chlorite 

 predominant over tourmaline. Cassiterite usually more than 

 wolfram. Sulphides common. Middle of tin-tungsten zone. 



3. Quartz-tourmaline-cassiterite lodes. Tourmaline predominant. 

 Wolfram absent. Sulphides absent or subordinate. Lower part of 

 tin-tungsten zone. Occurring in depth in the granite. 



It was also noted that if wolfram was present in the lode, copper 

 was absent or subordinate. The detailed microscopic characters of 

 the veinstones were given in tabular form at the end of the paper. 



CORRESPONDENCE. 



PHYSIOGRAPHIC RELATIONS OF LATERITE. 



Sir, — In reply to Professor W. M. Davis's inquiry in the September 

 number of this Magazine, I should in the first place explain that my 

 paper on Lateritization in Sierra Leone was concerned essentially with 

 the chemical and mineralogical changes undergone by the different 

 rocks of the country in the course of their transformation into 

 laterite, and only to a minor extent with the relation of the laterite 

 to surface forms and physiographic old age. Nevertheless, I am 

 indebted to Professor Davis for directing my attention to several 

 additional points of interest. 



In Sierra Leone, as elsewhere in the tropics, laterite is in general 

 best developed on surfaces of gentle slope or low relief, as the 

 following examples from different parts of the country will show : 



1. The Coastal Plain. — This plain, which along the inner margin 

 rises to a height of about 400 feet above sea-level, consists of 

 sediments, probably of Pleistocene age, which are still in process of 

 lateritization. Locally on these sediments there occur large bare 

 sheets of laterite, free from vegetation and soil, similar to those 

 described by Mr. Morrow Campbell,^ and explained by him as due 

 to rapid growth of highly ferruginous laterite ; this laterite 

 diminishes the fertility of the soil, which, losing the protection of 

 the dying vegetation, becomes gradually washed away by the 

 torrential rains. 



2. The Crystalline Rods of the Loivlands. — These rocks under the 

 following conditions are in general deeply kaolinized and lateritized— 



(a) Where rising as low hills above the sediments of the coastal 

 plain : 



(b) On those undulating areas in which the plain merges along its 



^ " Laterite, its Origin, Structure, and Minerals " : Mining Mag., vol. xvii, 

 1917, p. 178. 



