360 Br. R. D. Thomson on 



found in rocks connected with a red sand-stone, which is 

 deposited in horizontal strata along the hanks of the Tweed, 

 in the neighbourhood of Melrose, and appears to be re- 

 ferable to the old red sand-stone series, or transition for- 

 mation, being occasionally interrupted by dykes of green- 

 stone and clay-stone porphyry. In the latter of these trap 

 rocks, two of the minerals alluded to are found in con- 

 siderable quantity, while the third, seems to occupy a place 

 in the sand-stone itself. 



I shall describe their characters and composition, and 

 compare them with the simple compounds of silica and 

 alumina which have been examined. The first of these 

 minerals, I have termed Tiiesite,^ from Tuesis the river 

 Tweed. It occurs in veins in porphyry, or indurated sand- 

 stone slate, which is intimately connected with the old red 

 sand-stone. Its colour is milk white, opaque ; lustre dull ; 

 sectile. Hardness 2*5. Specific gravity from 2*434 to 2*558. 



Before the blow-pipe per se, it becomes blue and brittle, 

 fusing with carbonate of soda into an opaque bead, and with 

 borax and salt of phosphorus into a transparent glass. It 

 forms an excellent slate pencil. A portion of the mineral 

 was finely pulverized and fused with carbonate of soda. 

 The silica being separated in the usual manner, the alu- 

 mina was precipitated by caustic ammonia in the form of 

 beautiful white flocks, which after determining its weight, 

 was dissolved in sulphuric acid, with the addition of potash. 

 Regular crystals of potash — sulphate of alumina were 

 the result of the gradual evaporation of the solution. The 

 liquid remaining after separating the alumina was preci- 

 pitated by oxalate of ammonia. The product was a small 

 quantity of lime. The residual liquor was evaporated to 

 dryness, the dry salts heated to redness, dissolved in pure 

 water and boiled with carbonate of soda; a precipitate of 

 magnesia ensued. This precipitate being weighed, was 

 dissolved in dilute sulphuric acid. The whole of it dis- 

 solved, with the exception of a minute portion of silica, 

 scarcely appreciable, and which produced an amber coloured 

 bead when fused by the blowpipe with carbonate of soda.f 

 The water in one trial amounted to 13*5 per cent, in another 

 to 13*2. 



* See description, Thomson's Mineralogy, vol. i. 214. 

 t This is a peculiar state of silica well known to chemists as occurring in the 

 latter stage of analyses, which has frequently been mistaken for Titanic acid. 



