Correspondence — J. B. Scrivenor. 287 



a mixture of hydrous silicates of alumina were present in a clay,, 

 the resultant composition would be that of kaolinite, and to recalculate 

 as bauxite from a number of analyses, with the help of a mineralogical 

 slide-rule, the excess of alumina over that required for kaolinite. 

 The objections to this procedure were so clear that I was surprised to 

 read Professor Hies' detailed criticism in the June number of the same 

 periodical. 



Whatever one may think about laterite, whether the setting 

 property is an essential characteristic or not, whether the word would 

 exist but for that property or not, it is now clear that under the 

 heading 'laterite' penologists have taken up the study of hydrates 

 of alumina as weathering products. I think the relationship is 

 unfortunate, as may have been gathered previously, and that the 

 main objection to Dr. Fermor's classification of laterites and clays 

 (Geol. Mag., 1911, p. 514), apart from the two new terms, is the 

 necessity of long and difficult chemical analyses in place of deter- 

 mining the ' setting' property of laterite or plasticity of clay. 



I need not dwell on the dangers of mineral recalculations in very 

 fine-grained rocks where attempts at identification of component 

 minerals only reach what a distinguished petrologist, under whom 

 I once worked, called ' pious opinions ', nor need I recall the dangers 

 of assuming clays to be composed of kaolin. Mr. Hutchings' work 

 in the 1894 volume of this Magazine will be remembered by most of 

 those interested in the subject, and Professor Paes deals with the 

 matter in his Clays, their Occurrence, Properties, and Uses (pp. 40 

 et seq.). In dealing with weathering products recalculation from 

 a total analysis will not do, and the best method I have arrived at yet 

 is fractional treatment with acids and alkali solution. The idea is 

 simple. Alumina may be present in a weathering product as silicate 

 or hydrate, or perhaps as colloid alumina, in a soluble form (kaolinite 

 yields to prolonged acid treatment). If it is there as a silicate, 

 gelatinous silica will be liberated and perhaps go partly into solution. 

 Experiment with silica derived from powdered wollastonite by acid 

 treatment shows that a rapid wash with hot 10 per cent KHO (say 

 for 15 seconds) should take up all liberated silica without attacking 

 other minerals in the rock to a serious extent. If no silica, or very 

 little, is found in the KHO solution or the acid solution, then it is 

 a fair presumption that the alumina dissolved was not there as 

 a silicate or that there is only a slight admixture of silicate. 

 Successive acid attacks should give approximately the amount of 

 soluble hydrate (or colloid alumina) present. Will anyone tell me 

 of a better method for mixtures of hj'drous silicates and hydrates 

 where only chemical analysis can be employed? 



My impression so far is that aluminium hydrates are formed here 

 in small quantities wherever aluminium silicates exist, and that 

 kaolinite is decomposed to a certain extent. I have examined 

 kaolinized felspar crystals in this connexion, the kaolin being 

 a product of weathering, and believe them to contain some hydrate. 

 One chemist confirms my results. Another contradicts them. 



A point of some interest to me has cropped up lately in connexion 

 with the discussion about laterite in this Magazine. Dr. Evans said 



