Revieus — A. B. Scarlets History of Clay. 233 



YII. — The Natural HrsroEY of Clay. By Alfked B. Seaele. 8vo ; 



pp. viii, 176. Cambridge: University Press, 1912. Price Is. net. 

 rilHIS little work is issued as one of "The Cambridge Manuals of 

 JL Science and Literature ", and will be of most service to those 

 studying the practical uses of clay. As the author remarks, it is 

 difficult to give a definition of clay, especially when it includes such 

 diverse deposits as kaolin, fuller's earth, marl, loam, shale, slate, 

 fire-clay, and boulder-clay. He rightly decides that clay is "not 

 a mineral but a rock", irrespective of any legal significance of the 

 term 'mineral',^ but ultimately he remarks (p. 150) that "In so far 

 as it can be isolated true clay appears to be an amorphous, or practically 

 amorphous, material which may under suitable conditions crystallize 

 into rhombic plates of Kaolinite ". 



He gives an account of the colours, chemical composition, and 

 physical characters of various clays, but finds fault with the geologists' 

 use of the term 'boulder-clay'. He would like the term "restricted 

 to the plastic portions and not including pockets of sand, gravel, and 

 other non-plastic materials". In practice he would soon find the 

 impossibility of any such restriction. He gives an analysis of boulder- 

 clay, presumably of the clay matrix, but the subject reminds us of 

 a chemist who asked a geologist about an analysis of boulder-clay, 

 and the reply was, "you might as well ask for the analysis of 

 a house." We had not heard of certain Palaeozoic or Primary Shales 

 being termed ' marls ' when soft and friable, nor are the Thames 

 Yalley brickearths as a rule 'natural marls \ The geological portions 

 of this work are weak in many respects. The most typical deposits 

 of Kimeridge Clay are not in Huntingdonshire, the ' Kellaway blue 

 claj^s' are not of fi'eshwater origin, nor are the differences in the 

 Wealden clays "more in the fossils occurring in them than in the 

 characters of the clays themselves ", they depend mainly on strati- 

 graphical position. 



In another edition the Table of "Chief Clay Rocks (arranged 

 geologically)" should be amplified, as "Oolitic (brick and tile 

 clays)" and "Cretaceous (cement clays, brick clays)" contain no 

 useful information ; Oxford Clay, Gault, etc., should be given. 

 Elsewhei'e in the volume more stratigraphical particulars are recorded, 

 but they are meagre. Watcombe terra-cotta clay should have been 

 mentioned among the Permian clays, and there is some confusion 

 (pp. 54, 131) in the references to Triassic marls, and the Etruria 

 Marl Series in the Upper Coal-measures of North Staffordshire. 



VIII. — A Geological Excursion Handbook for the Bristol District. 

 By Sidney H. Reynolds, M.A., F.G.S., Professor of Geology in 

 the Universitj^ of Bristol. 8vo ; pp. 224, with 50 text-illustrations. 

 Bristol, J. W. Arrowsmith, Ltd.; London, Simpkin, Marshall & Co., 

 Ltd. Price 3s. &d. net. 



WE do not doubt that this little volume will prove a useful and 

 very instructive guide to all interested in the geology of the 

 country around Bristol. The area is naturally limited on the west by 

 ^ See decision respecting the question "Is China Clay a Mineral? ", Geol 

 Mag., 1908, p. 564. 



