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V.—The Work of Professor Lacroix on the Laterites of French Guinea

Published online by Cambridge University Press:  01 May 2009

L. Leigh Fermor
L. Leigh Fermor
Affiliation:
Geological Survey of India
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Extract

A short while ago I received from Professor A. Lacroix a copy of a recently published memoir on the laterites of French Guinea, accompanied by the request that I should, if possible, give an account of this work in the Geological Magazine, “puisque c'est là qu'ont paru les articles les plus importants sur cette question.” I have undertaken this task with the greater pleasure because Professor Lacroix was so good as to show me, when I was passing through Paris recently, both his hand-specimens and his microscopic preparations of these rocks, and because the memoir in question seems to me to be one of the most important and thorough pieces of work on laterite published since Max Bauer's account of the laterite of the Seychelles. Although Professor Lacroix' memoir deals professedly with French Guinea only, yet it is probable that many of the conclusions will be found to apply to other regions, and on this account, and because the series in which the memoir appears will probably not be readily accessible to all, I have thought it desirable to expound it at some length.

Type
Original Articles
Information
Geological Magazine , Volume 2 , Issue 1 , January 1915 , pp. 28 - 37
Copyright
Copyright © Cambridge University Press 1915

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References

page 28 note 1 Neues Jahrb. für Min., etc., ii, pp. 192–219, 1898.Google Scholar

page 29 note 1 Arch. néerl. Sc. exactes et nat., xv, pp. 294–305, 1910.Google Scholar

page 29 note 2 Neuss Jahrb. Min. u. Pet., Festband, 1907, pp. 33–90.Google Scholar

page 29 note 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.

page 30 note 1 “What is Laterite?”: Geol. Mag., n.s., Dec. V, Vol. VIII, p. 514, 1911.Google Scholar

page 30 note 2 There seems to be no good reason why we should exact less precision in naming products with less than 50 per cent of lateritic constituents than with products containing more than 50 per cent. Indeed, greater possibilities of variation amongst the former group necessitate greater precision rather than less.

page 30 note 3 The undesirability of this course is indicated by consideration of the case of latérite d'alluvions noticed in Section IV.

page 30 note 4 This can be regarded only as a general statement, for examples of the crystalline and colloid forms occurring together are given later in the memoir.

page 30 note 5 Min. de la France et de ses colonies, iii, p. 342, 1901.Google Scholar

page 30 note 6 Throughout this memoir the author uses the term hydrargillite rather than gibbsite, but in the adjectival form he prefers gibbsitic as being less cumbrous.

page 31 note 1 This nomenclature appears at first sight to be heterogeneous, because gibbsite is a mineral and bauxite a rock (according to Lacroix himself); but this use of bauxitic must be taken as analogous to that of lateritic, and as referring to the presence of bauxitic constituents. It is doubtful if it will be possible thus to restrict the use of the term bauxite outside the realms of mineralogy and petrology, because for economic purposes we have available no other inclusive term, except aluminous laterite or aluminium-ore.

page 31 note 2 N.J. Min. u. Pet., Festband, 1907, p. 87.Google Scholar

page 31 note 3 “Laterite in Western Australia”: Geol. Mag., 1912, p. 400.Google Scholar

page 32 note 1 Thus in the publications of the Geological Survey of India it is used by W. T. Blanford as early as 1859; see “Note on the Laterite of Orissa” (Mem. Geol. Surv. Ind., i, p. 286). Blanford clearly distinguishes between the overlying laterite and the underlying lithomarge produced by the decomposition of gneiss. He writes: “The underlying form varies so much in constitution according to the rock from which it is derived, that scarcely any petrological term will include all its varieties. As a rule, however, it is, when derived from gneiss or other felspathic rock, a more or less ferruginous clay varying in purity. To such substances the name Lithomarge has frequently been applied, and it seems the most applicable in the present instance, it being understood that all the impure varieties derived from quartzose metamorphic rocks or sandstone are here included in the term.” He notes the “apparent passage of Lithomarge into Laterite”, but gives a different explanation from that of Professor Lacroix.

page 32 note 2 By using the term lithomargic where Lacroix uses argileuse, we have the adjective argillaceous or argileuse, available as a comprehensive term, including both ‘kaolinic’ and ‘lithomargic’.

page 32 note 3 I prefer the term lateritic constituents (L.C.) to hydrates, because, at the surface, laterite sometimes contains the anhydrous mineral hematite, developed as a result of dehydration.

page 32 note 4 In which, by way, the author overlooks Mr. J. Morrow Campbell's paper entitled “The Origin of Laterite” (Trans. Inst. Min. Met., xix, pp. 432–57, 1910), based largely on observations made in Haute Guinée.

page 33 note 1 Mineral. de la France et de ses colonies, iii, p. 342.Google Scholar

page 33 note 2 Zeitsch. Chem. industr. Kolloide, iv, p. 90, 1909 (Lacroix).Google Scholar

page 33 note 3 Neues Jahrb., Beil. Bd. xxxiv, p. 518, 1912.Google Scholar

page 33 note 4 Centralblatt f. Min., 1912, pp. 19 and 104; and 1913, p. 193.Google Scholar

page 33 note 5 Zeitsch. prakt. Geol., xxi, p. 545, 1913 (Lacroix).Google Scholar

page 35 note 1 I have retained the term diabase throughout this article, although personally I think it an unnecessary term, the word dolerite being the more desirable.

page 35 note 2 In a footnote Lacroix remarks that the essentially kaolinic transformation of a diabase is an exceptional case that he has not himself observed. Such cases occur, however, in India: cf. Mem. Geol. Surv. Ind., xxxvii, p. 376, 1909, where is described the occurrence at Yeruli of a zone of variegated argillaceous material intervening between the underlying basaltic rock and overlying laterite.

page 36 note 1 Mr. E. S. Simpson (Geol. Mag., 1912, p. 401) has criticized adversely my postulation (Geol. Mag., 1911, p. 459) of two distinct modes of decomposition of rocks resulting respectively in the formation of clay and laterite, apparently because in Western Australia primary laterite is always found to overlie an almost pure pipeclay and this in turn a crystalline rock. From the work of Lacroix it is seen that the felspar in a crystalline rock, such as nepheline-syenite, may alter directly into gibbsite or directly into a hydrous aluminium silicate. When the latter forms, a chemical change may cease at this stage, as in Los Archipelago, or it may proceed further, as in the case of the mica-schists noticed on a later page in this paper. With reference to Mr. Simpson's comparison of lateritization with the formation of saline efflorescences, Lacroix' work indicates a marked distinction. A saline efflorescence is deposited on the surface of the ground; the constituents of a lateritic crust, on the other hand, are partly residual (although recrystallized), formed approximately in their present place from the once fresh primary rock, at the time when the zone of leaching was at this level; and have partly been brought up from the zone of leaching and added to the residual portions: in any case these constituents were deposited inside the crust and not on the surface, except perhaps to a very minor degree. In the following paragraph Mr. Simpson gives a curious definition of a replacement of a rock, namely, “a deposit accumulating in the actual space originally occupied by the solid rock which has yielded the materials which compose the laterite.” Surely the essential feature of replacement is that the replacing mineral has been brought from elsewhere, and has been deposited in the place of another mineral removed in solution.