362 Dr. R. D, Thomson on 



mixed with galena, carbonate of lead and calamine, which 

 traverse a transition limestone, in masses varying from the 

 size of the fist to that of a cubic metre. Two specimens 

 afforded,* 



99 99-5 

 Their formula is, 



2 Al S + Al S2 -f 5 Aq. 

 If then the lime and magnesia be considered foreign to 

 the composition of tuesite, we shall have halloysite forming 

 with it a sub-species of the same mineral, but possessing 

 decidedly distinct characters, apparently occasioned by the 

 addition of 3 atoms of water. And it seems necessary that 

 the term halloysite should be restricted to those compounds, 

 consisting of silica and alumina, in the proportion of 4 

 atoms to 3, with 5 atoms of water, while tuesite contains 

 the same proportions of solids but possesses only 2 atoms 

 of water. 



2. Bisilicate of Alumina or Fuller s Earth. — This mineral 

 is found in round masses in the bed of a stream, associated 

 with clay-stone porphyry, near Maxton. Sp. Grav. 2*394. 

 With nitre, soda and salt of phosphorus, fuses before the 

 blowpipe into an opaque mass. With borax, fuses into a 

 transparent bead, pale amber coloured when hot, colourless 

 when cold. Colour yellowish white ; fracture earthy, soft, 

 soiling the fingers ; scratched by the nail, tuesite and sul- 

 phate of lime ; adheres to the tongue like halloysite ; con- 

 tains crystals of decomposing felspar interspersed through 

 the mass. Its constituents I ascertained to be. 

 Silica, . . . 57-105 4 atoms. 

 Alumina, . . 31*850 2 „ 

 Magnesia, . . 2*615 

 Water, . . . 7*280 1 „ 



98*850 

 and its formula 2 Al S^ + Aq. 



* Traite des Essais, i. 5B. 



