372 Reviews — J. Allen Howe — Kaolin, China-clay, etc. 



by which the felspars of solid granitic masses have been converted 

 into friable, white kaolin, though possibly sufficient in a few cases, 

 has long been recognized as incomplete, especially by those acquainted 

 with the deposits of Cornwall and Devon. Two or three facts may 

 be mentioned here, of which the ordinary theory of ' surface 

 weathering' does not seem to afford a satisfactory explanation. 

 For instance, the existence of extensive tracts of unaltered granitic 

 rock in many localities at once indicates that the decomposing action 

 of water and carbon dioxide on felspar is not universal, and it would 

 not seem unfair to throw on the advocates of this theory of kaolini- 

 zation the onus of explaining what factors have been operative in 

 causing this selective action on the part of the decomposing agents. 

 That kaolin would be easily and quickly removed by the ordinary 

 agents of denudation is not a sufficient explanation, for then such 

 granitic masses as those referred to would show signs of erosion where 

 the kaolinized material had been removed. From a purely chemical 

 point of view, too, the ordinary theory of weathering does not make at 

 all clear by what mechanism two molecules of chemically combined 

 water became an integral part of the argillaceous compound. A satis- 

 factory explanation of this process may reasonably be demanded of 

 any theory of kaolinization, especially in view of the fact that water 

 can only be reintroduced into a dehydrated clay with the greatest 

 difficulty, even if at all. At the same time it must be frankly admitted 

 that the chemistry of the conversion of felspar into kaolin is very 

 complex, and it is perhaps not too much to say that no theory of 

 kaolinization at present propounded affords a complete explanation 

 of all the reactions involved. There seems no doubt that during the 

 decomposition of the original rock a number of secondary minerals 

 had been formed, and consequently a number of secondary chemical 

 reactions must have occurred, which only increase the difficulty of 

 tracing the stages by which felspar has been converted into kaolin. 

 The depth to which the deposits extend and the improvement in 

 quality — in other words the more complete kaolinization — which is 

 often found with increasing depth, are phenomena that certainly lend 

 strong support to the theory that in such areas the action has passed 

 from below upwards. The advocates of particular theories have 

 perhaps been over ardent in support of their own pet theory and 

 prone to disregard the value of others. Certainly a perusal of the 

 evidence given in these pages, supported as it is by copious references 

 to original papers, is enough to show that the problem cannot be said 

 to be solved, and it was a happy inspiration which led the author of 

 the handbook to conclude his summary of the theories with the 

 quotation from F. W. Clarke : " Kaolin, like many other substances, 

 may be formed by any one of several processes, in all of which water, 

 hot or cold, and carbonic acid take part. No one interpretation can 

 fit all its occurrences." 



The details of composition and various physical properties of china- 

 clays from many localities, in chapter viii, together with the statistics 

 and list of china-clay works in England, are most useful additions, 

 brought together in compact form, so far as we can remember, for the 

 first time. 



