364 Dr. R. D. Tliomson on 



marge in composition, magnesia taking the place of the 

 potash. 



Kaolin from St. Tropez, 3 Al S^ 4- M S^ -f- 2 Aq. 

 Kaolin from Mende, 4 Al S^ + M S^. 

 For the sake of greater precision, I shall give a view of 

 the composition of the various Halloysites and Kaolins 

 analyzed by Berthier and Boussingault, including along 

 with them the minerals analyzed by myself, and expressing 

 their constitution by formulae. 



1. ] . Tuesite, 2 Al S 4- Al S^ h- 2 Aq. 



Under this species are comprehended Halloysite from 

 Guatequa analyzed by Boussingualt, and two Kaolins from 

 St. Yriex and Schneeberg, examined by Berthier, and Clay 

 of Angleur. 



2. Halloysite, 2 Al S + Al S^ +5 Aq, 



including the Halloysites of Anglar and Hall, and, perhaps, 

 Nontron. 



II. Bi-silicate of Alumina or Fuller's earth, 



2 Al S^ + Aq, and Al S^ -f Aq, 

 expressing the composition of Halloysite from Fahlun, 

 Kaolin from Normandy, and Fuller's earth from the Tweed. 

 Kaolin from Meissen possesses less water = 3 Al S" + 2 Aq. 



III. Linzinite is the name which John originally gave 

 to the Halloysite of Hall. It may, with propriety, be 

 applied to the ter-silicates of alumina, whose composition 

 is denoted by 



Al S3 + 3i Aq. 

 Berthier analyzed two Halloysites from St. Sever and 

 Confoleus possessing this constitution, besides clays from 

 Hoeganas, Forges, Montereau, Cymolite, Cologne, with 

 less water. 



IV. Lithomarge, 8J Al S^ -f- K S^ + 4 Aq. 



Under this species, we may, perhaps, include the Kaolins 

 from St. Tropez and Mende whose expressions have been 

 already exhibited. 



In presenting the preceding arrangement, it is only my 

 intention to assist in simplifying the study of these interest- 

 ing compounds ; for it appears more beneficial to classify 

 minerals, not according to theoretical views, but according 

 to their actual nature. M. Berthier, whose analytical 

 accuracy none will dispute, has thrown all the simple 



