454 Dr. L. Leigh Fermor — What is Laterite ? 



was a dual one. The large tubercles become gradually restricted to 

 the adoral surface, and often reduced in number, while the actual 

 density of the tuberculation increases. This increase is attained, 

 either by a simple multiplication of the normal vertical series, as in 

 Discoidea (this being sometimes associated with the transformation of 

 granules to tubercles at the ambitus), or by a more uniform distribution 

 of those series over the area of the plates, and the frequent substitution 

 of two tubercles in the place of one, as in Conulus. Both these 

 types of change lead away from the primitive, ' Regular ' condition 

 of tuberculation, and trend towards the Clypeastroid and Cassidulid 

 arrangement. 



In view of the fact that, in almost every detail of comparative 

 anatomy, the Holectypoida show a retarded evolution, it may be 

 considered that the sequence of changes reviewed above actually 

 represents, in a dilatory and extended manner, the process which, at 

 one or more stages of geological history, produced a uniform and 

 packed tuberculation from the solitary series of large tubercles which 

 characterize the Cidaridae. 



Finally, I wish to express my thanks to Messrs. G. E. Dibley, 

 LI. Treacher, and T. H. Withers for the loan or gift of specimens 

 which were invaluable in the construction of the diagrams. 



III. — What is Laterite? 

 By L. Leigh Fermor, A.K.S.M., D.Sc, F.G.S., Geological Survey of India. 1 



Contents. 



I. Introduction. 



II. The Previous Discussion. 



III. The Nomenclature of Laterite. 



IV. The Application of the term Bauxite. 

 V. The Lateritic Earths of British Guiana. 



VI. Summary. 



I. Introduction. 



t\EW natural mineral products have aroused more general interest 

 or been more provocative of discussion amongst geologists than 

 that superficial rock-formation so typical of the tropics known as 

 laterite. This material excites interest not only because of its 

 chemical composition, but also on account of its wide distribution. 

 It has been recorded, for instance, from tropical South America 

 (e.g. the Guianas and Brazil), Central Africa (e.g. Guinea and East 

 Africa), the Seychelles, India, the Malay Peninsula, the East Indies, 

 and Western Australia. Many papers have been published dealing 

 with its distribution, composition, and also its origin, to explain 

 which many hypotheses have been invented. 



Owing to the large number of occurrences of manganese-ore found 

 in India in association with lateritic rocks, 2 I had, when working at 



1 Published with the permission of the Director, Geological Survey of India. 



2 To prevent this casual allusion to the subject of Indian manganese-ores 

 giving rise to one of those misconceptions which, once formed, are so difficult 

 to eradicate, I must state at once that the lateritic manganese-ores are, as a rule, 

 of small importance compared with those of the Archasan areas. 



