L. Leigh Fermor — Laterites of French Guinea. 123 



a form in which Barrande could not discern the ornament so 

 characteristic of Plumulites. It is even impossible to make out the 

 contour of the plates, or their number, which J. M. Clarke thought 

 composed only two rows, so this form is very probably distinct from 

 the genus Plumulites. The state of preservation of all the examples 

 of P. follieulum as contrasted with the other forms would suggest 

 this. There is no definite evidence that the single plate referred by 

 Barrande (1872, pi. xx, fig. 10) to this species really does belong 

 to it. 



In conclusion, it may be said that we know very little of the 

 relationship to one another of Lepidocoleus, Plumulites, and Turrilepas, 

 and very little is known even of the structure of their shells. In 

 none do we know the relation of the animal to the shell, and except 

 for the ornamentation and the downward growth of the plates there 

 seems to be no other character advanced in favour of their reference 

 to the Cirripedia. 



I wish to express my thanks to Dr. F. A. Bather and Dr. W. T. 

 Caiman for their valuable assistance with this paper. 



IV. — The Work of Professor Lacroix on the Laterites of 



French Guinea. 



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



(Concluded from the February Number, p. 82.) 



Alluvial Laterites or Later itites f Laterites alluvionaires) . 



,4 LL the phenomena hitherto noticed have been worked out for the 

 f\_ case of rocks altered in situ, but they also take place in the 

 products of transport resulting from the demolition of laterites in situ. 

 In these lateritic alluvia (alluvions lateritiques) or lateritites the 

 zone of departure is the base of the alluvium itself, in which the 

 hydrolysis of any aluminous silicates still remaining is continued, 

 whilst the upper part forms a ferruginous cuirass, differing from 

 that of the non-transported laterites only when quartz debris or 

 transported fragments of rock, themselves usually lateritized, are 

 present. When the alluvium is constituted by fine particles the 

 conditions are favourable for the formation of very regular ferruginous 

 pisolites, and in Professor Lacroix' opinion the specimens of constant 

 aspect so frequently seen in collections from many tropical countries 

 probably come from such occurrences. 



Lacroix adopts for these rocks my term lateritite, proposed for 

 detrital or secondary laterites, which, it is seen above, are subjected 

 to a continuance of the same processes that led to the formation of 

 the primary laterite. (See analysis No. 16, Table II, infra, for a very 

 impure lateritite with 31 per cent quartz, 30 per cent clay, and 

 39 per cent lateritic constituents.) He also finds in many parts of 

 Guinea a conglomerate composed of small fragments of gihbsitic 

 laterite, mixed with clastic products, in particular quartz, derived 

 from neighbouring rocks, and constituting a true sedimentary rock 

 intermediate between the type described in the preceding paragraph 

 and the laterite d 'alluvions to be noticed in the next paragraph. 



