Reviews — S. W. Williston — Water Reptiles. 37 



is here preserved through a very great thickness of altered rock ; but the 

 degree of cohesion of the rock is profoundly modified, the rock being 

 unctuous to the touch and friable when dry. The micas are pro- 

 gressively deprived of their alkalis concurrently with the fixation of 

 a larger and larger quantity of water (see analysis on p. 306 of 

 Lacroix' memoir). From this results kaolinite or colloidal silicates 

 of aluminium, mixed with a greater and greater quantity of aluminium 

 hydrate. This mode of alteration also characterizes certain granites 

 and gneisses in Guinea. None of these products of alteration in the 

 zone of leaching would be designated laterite ; they are fundamentally 

 kaolins or clays, passing upwards into lateriiic clays. 



The type of alteration is not characteristic of a given rock. — Judging 

 from the results given above, one might think that in the Tropics 

 the mode of alteration of the rocks is always determined by the 

 mineralogical and chemical composition of the original rock. Such 

 a generalization would be inexact, as is shown by observations made 

 in Madagascar, where the gabbros, diabases, and nepheline-syenites 

 behave as in Guinea, but where the granites and gneisses give rise 

 not only to a great development of argillaceous alteration products, 

 as in Guinea, but also in places give, rise to gibbsitic laterites, often 

 quite free from iron. 



(To be continued.) 



REVIEWS. 



I. — Watek Reptiles oe the Past and Present. By Professor 

 S. W. Williston. pp. 251, with 131 text-figures. Published 

 by the University of Chicago Press : Agents in Great Britain, 

 the Cambridge University Press, Fetter Lane, London. Price 

 12s. net. 



IN this volume Professor "Williston has endeavoured to give a popular 

 account of the various groups of reptiles which have become more 

 or less adapted to an aquatic life, and to a great extent he has 

 succeeded in this difficult task. At the same time it is not easy to 

 tell how far a work of this kind will appeal to the general reader, 

 since much that is highly technical and unfamiliar necessarily 

 remains. In any case, the book is a valuable one because it brin»s 

 together in a compact form a large amount of information hitherto 

 widely scattered and often not easily accessible. 



In the earlier chapters the author gives a general account of the 

 mode of occurrence of fossil reptiles, of their classification and their 

 osteology. The chapter dealing with the last-mentioned subject 

 might perhaps have been extended with advantage, and, indeed, if 

 so good an authority as Professor Williston would publish a volume 

 on the osteology of the class, it would fill one of the most notable 

 gaps in palseontological literature. The subject essentially is one 

 for the palseontologist, since in perhaps no other group of the 

 Yertebrata are the fossil forms of such preponderating importance. 

 This will be understood when it is realized that out of the fifteen 



