406 Dr. T. Thomson on the Minerals 



The colour is light brownish yellow, and, like phosphate of 

 lead, it is crytallized in six-sided prisms. 



My specimen was analysed a good many years ago by my 

 nephew, Dr. R. D. Thomson, who found it a compound of 

 1 atom of chloride of lead, and nearly 9 atoms of sesquiva- 

 nadiate of lead *. 



Besides these ten species of lead ore which occur at Lead 

 Hills or Wanlock Head, there are found also native copper 

 and malachite, or hydrous dicarbonate of copper ; a beautiful 

 green-coloured mineral often crystallized, and when large 

 specimens occur, as the well known ores from Siberia, they 

 are often cut into ornamental tables, &c. 



The Lead Hills malachite is rather a dark green, but the 

 specimen of it subjected to analysis was very nearly pure. 



Lead Hills also contains specimens of blende or sulphuret 

 of zinc. By far the finest specimens of hydrous silicate of zinc 

 which I have seen, are from Lead Hills ; they constitute beau- 

 tiful white crusts or plates. Its specific gravity is 3*1645, and 

 its constituents, as determined by analysis, are 



Silica ' 23'2 1 



Oxide of zinc 66*8 1*12 



Water 10*8 0*82 



100-8 

 or it is a simple silicate of zinc with rather less than an atom 

 of water. It is curious that in all the analyses of this mineral 

 hitherto made the water never amounts to an atom. In that 

 made by Mr. Smithson the water was only 4^ per cent. By 

 Berzelius's analysis in 1819, it amounted to 7|- per cent., and 

 in one analysis of a specimen from Lead Hills it was 10'8 per 

 cent., or almost 1 1 per cent. The probability is that it un- 

 dergoes a kind of efflorescence by keeping, and that originally 

 the water in the ore was exactly an atom. If that supposition 

 be correct, the water originally must be 13 ~ per cent. 



The Kilpatrick hills, which bound the valley of the Clyde 

 from the Stockey muir to Dumbarton, and also the corre- 

 sponding but lower range on the south side of the valley are 

 composed of various trap rocks, among which amygdaloid 

 is pretty common. Now amygdaloid is a rock full of almond- 

 shaped cavities of various sizes, usually filled up by crystal- 

 lized minerals, many of which, though not the whole, belong 

 to the beautiful tribe of zeolites. We ma}', therefore, divide 

 the minerals found in these mountains into two sets: 1. The 

 zeolites, so called because they froth before the blow-pipe, 

 and they owe this frothing property to the great quantity of 



* Records of General Science, vol. i. p. 34. 



