L. Leigh Fermor — Laterites of French Guinea. 29 



Professor Lacroix followed the railway line, which provided many- 

 valuable sections ; then, after examining the cuttings of a line under 

 construction from Kouroussa to Kankan, he descended the Niger to 

 Siguiri, which is about 360 miles in a straight line from Konakri. 

 After touring in Boure, a part of the ancient Sudan, he returned to 

 Siguiri by an affluent to the left bank of the Mger, the Tinkisso, 

 returning thence to Konakri. Professor Lacroix has also made a field 

 study of the laterites of Madagascar on the opposite side of Africa, 

 but he is compelled by lack of space to defer to a subsequent volume 

 of the Archives an account of his work in that island. 



The importance of this work on Guinea results in the first place 

 from the fact that the author was able to examine in the field a large 

 number of fresh sections, many of which showed the complete passage 

 from the underlying fresh rock, through the various stages of 

 lateritization, to the surface crust or cuirasse ; that secondly he 

 collected his specimens with judgment from known points in the 

 sections, and thirdly that he has supplemented the chemical analyses 

 (due to M. Boiteau) by careful microscopical studies of the 

 mineralogical composition. He is thus able to trace the successive 

 stages of change in both chemical and mineralogical composition from 

 underlying fresh rock to surface crust. His results are rendered the 

 more convincing by a splendid series of plates showing the field 

 relationships (twelve photos), the aspect in hand-specimens (twelve 

 photos), and, finally, the microscopic appearance (twenty-four 

 photos). 



II. Nomenclature. 



In an introductory chapter Professor Lacroix discusses the 

 nomenclature of the subject. After referring to Buchanan's original 

 definition and the demonstration of Van Bemmelen ! and Max Bauer - 

 that laterites rich in hydrates sometimes contain a considerable 

 quantity of silicates of aluminium, without differing in any way in 

 external characters from laterites quite free from silicates, 3 he 

 concludes (p. 258) — 



" Enfin, de veritables argiles, de vrais kaolins, abondent sous les tropiques, 

 et ils sont, eux aussi, souvent colores en rouge par de l'oxyde de fer." 



The author then arrives at the following definition of laterite 

 (p. 259):— 



" Les produits de decomposition de toutes les roches, silicatees alumineuses 

 caracterises, au point de vue chimique, par la predominance de hydroxydes 

 d' aluminium et de fer, avec generalement de l'oxyde detitane, apres elimination 

 plus ou nioins complete des autres elements de la roche fraiche : alcalis, chaux, 

 magnesie et silice " : 



this definition agreeing with that now generally accepted. 



1 Arch, nierl. Sc. exactes etnat., xv, pp. 294-305, 1910. 



2 Neues Jahrb. Min. u. Pet., Festband, 1907, pp. 33-90. 



3 For the frequent difficulty or impossibility of distinguishing laterites from 

 some clays in the absence of chemical examination, see Sir T. H. H. Holland, 

 in the discussion on Mr. J. M. Campbell's paper on "The Origin of Laterite ", 

 Trans. Inst. Min. Met., xix, p. 455, 1910, and L. L. Fermor, Geol Mag., 

 1911, p. 510. 



