REVIEWS 169 



surrounded by groups of primary xylem. But the peculiarity of the fossil lies in 

 the structure of the secondary wood, which consists of ten or more successive 

 zones ; the zones alternate, the elements of each zone running in a direction 

 approximately at right angles to those of its neighbours. This has the curious 

 effect that the transverse and radial sections appear just alike — every alternate 

 zone being cut radially in transverse section, and transversely in radial section, and 

 vice versa. All the zones, in spite of the violent changes in their direction, are 

 quite continuous, and appear to be the product of a single cambium. The remark- 

 able structure, which seems to be without analogy (except, very remotely, with 

 Dracanci), is beautifully illustrated, and the facts quite clearly established. The 

 tracheides are scalariform. The plant was undoubtedly a Cycadophyte, but there 

 is no clue to its closer affinities. 



A much fuller review would be needed to do complete justice to this important 

 work, dealing, as it necessarily does, with so many matters of detail. The illus- 

 trations, both plates and text-figures, are admirable, and these two volumes, which 

 we hope to see followed by others, are a most valuable addition to the great series 

 of catalogues published under the auspices of the British Museum. 



D. H. Scott. 



MetamorpMc Geology. By C. K. Leith and W. J. Mead. [Pp. xxiii + 337, 

 with 16 plates and 35 figures.] (New York: Henry Holt & Co., 191 5. 

 Price $2.50.) 

 In this book "metamorphism " is taken to include all the changes— physical, 

 chemical, and mineralogical — that a rock may undergo from the time of its 

 original crystallisation as a primary igneous rock. As a broad basis for systematic 

 discussion the conception of a " metamorphic cycle" is introduced. Starting 

 with igneous rocks as the primary source and initial condition of the working 

 materials, the gradual evolution of the various end products of katamorp/iism 

 (weathering) is traced. The effects of cementation are then followed, and finally, 

 the development of slates, schists, and gneisses from any of the pre-existing types 

 is described. Changes brought about by cementation, or by dynamic and 

 thermal metamorphism, are grouped together as anamorphism. 



The work is divided into four parts, dealing respectively with Katamorphism, 

 Anamorphism, General Discussion, and Laboratory Work. Graphic methods are 

 used wherever possible, and indeed nearly all the illustrations are diagrams 

 illustrating the chemical and mineralogical changes undergone by average or 

 specific types of rocks. Generally these diagrams are clear and effective, but 

 occasionally the authors so overburden them with detail that it becomes a severe 

 mental strain to grasp their significance. Only four photo-micrographs are 

 reproduced, and field occurrence is left unillustrated. The illustrations thus 

 reflect both the purpose and the limitations of the book, which is almost wholly 

 devoted to a statistical method of investigation. Nevertheless, though " office " 

 study predominates over laboratory and field work, there is no lack of stimulating 

 suggestion, and one feels that the book marks a forward step in the direction of a 

 new geology which will be largely quantitative. 



In Part I there is an interesting chapter on laterites and bauxites. It is held 

 that kaolinisation is an intermediate process, and that if the resulting product be 

 porous, so that solutions can continue to percolate, then a further leaching of 

 silica takes place until only stable hydrated oxides are left. It is worthy of notice 

 that while many writers have assumed the average igneous rock to be equivalent 

 to approximately equal parts of granite and basalt, Messrs. Leith and Mead show 



