48 Correspondence — Dr. L. Leigh Fermor. 



But there are others resting directly on aluminous laterite. or even bauxite 

 that are practically, if not entirely, free from lateritic constituents ; prominent 

 among these are the soils at Akyma and at Christianburg, both of which are 

 sedentary soils resting on beds of bauxite. ' ' 



This correction invalidates my criticism on p. 562 (loc. cit.) to a large 

 extent, and many of the products which I suggested should only be 

 termed clay, soil, or sand, with or without the adjective 'lateritic ', 

 are obviously argillaceous and siliceous laterites. • 



Later in his letter Professor Harrison writes : — 



" The specimens I described in the paper were all collected from low 

 altitudes — below 500 feet — whilst many of them were from altitudes 20 to 

 180 feet only above sea-level. There are, however, as shown by C. B. Brown, 

 during his geological survey of the colony, vast areas of the higher lands of 

 British Guiana, 2,000 to 5,000 feet in altitude, covered with layers of con- 

 cretionary ironstone gravels of lateritic origin. 



' ' Becently an extended survey for railway purposes has shown that a vast 

 area of British Guiana in altitude from 500 to 1,500 feet is covered by a 

 ferruginous laterite which in composition corresponds to your definition of 

 ' Typical Laterite ', but which we have always termed ' ironstone '. It justifies 

 from your point of view van Capelle's description of another part of Guiana as 

 ' le pays de la laterite par excellence '." 



In view of these remarks of Professor Harrison, it cannot be 

 doubted that there are in the Guianas wide spi'eads of laterite. One 

 feature in which the laterites of Guiana tend to differ markedly from 

 those of India is the frequent presence of secondary quartz, which has 

 been observed both macroscopically and microscopically by Professor 

 Harrison in lateritic products derived from basic rocks originally 

 containing little or no free quartz. This secondary silica is of such 

 importance in places that it has segregated into quartz veins and 

 reefs, which are sometimes auriferous (see pp. 446, 447, 489 of 

 Harrison's paper). Many other authors, quoted en p. 490 of Professor 

 Harrison's paper, have described auriferous quartz of secondary 

 origin in the Guianas, e.g. Du Bois (loc. cit., pp. 21, 22), so that it 

 seems impossible to doubt that frequently in the Guianas the process 

 of lateritization has not been pushed to a finish owing to the non- 

 removal of at lenst a portion of the silica of the original rocks. This 

 may be due to the fact that, according to Harrison (loc. cit., p. 560), 

 " in the dense forests of the Guianas there may be said to be a perpetual 

 wet season, as under the shade of the trees, even during periods of 

 comparative drought, the land is invariably wet and more or less 

 soaked with water containing organic acids in solution." This last 

 passage suggests that the ground-water level is very close to the 

 surface, thereby rendering difficult the thorough drainage of the 

 decomposing rocks. If this be so, then it seems as if the process of 

 lateritization requires for its completion some condition, such as the 

 alternation of wet and dry seasons so characteristic of many tropical 

 lands, that will facilitate the periodical drainage from the soil of its 

 contained solutions. 



It would be interesting to learn whether there are in British 

 Guiana any masses of laterite entirely free from secondary quartz, 

 and, if so, whether such occurrences can be correlated with and 

 explained by local topographical and climatic conditions. 



L. Leigh Feemok. 



