414 Dr. T. Thomson on the Minerals 



But it contains an additional atom of bisilicate of alumina 

 and of water. 



10. Stilbite and Heulandite. — These two species are found 

 in great abundance in the Kilpatrick hills and at Carberry in 

 the valley which divides the Campsie hills from the Kilpatrick 

 hills. They are always, or almost always, of a deep flesh-red 

 colour, and generally crystallized, though seldom in very re- 

 gular shapes. These two species were first distinguished 

 from each other by Werner, who gave to stilbite the name of 

 radiated zeolite, and to Heulandite that of foliated zeolite. 

 Haiiy confounded the two under the name of stilbite. Mr. 

 Brooke pointed out the difference between them in 1822 ; 

 and left Haiiy's name stilbite to one of the species, while the 

 other was named Heulandite, in honour of Mr. Heuland of 

 London. It is unlucky that the names had not been reversed. 

 The word stilbite (from the Greek (tti\/3co, to shine) was ap- 

 plied by Haiiy, in consequence of the great lustre of the 

 mineral. Now the base of the prism in Heulandite has a 

 much greater lustre than in stilbite, being pearly and splen- 

 dent. In the specimens from this neighbourhood, the lustre 

 is not so great as in those from the Feroe islands, which are 

 white, and have the pearly lustre in perfection. 



The constituents of both these minerals are silica, alumina, 

 lime and water. 



Heulandite contains most silica and least water. 



Stilbite is 2 Al S 3 + Cal S 9 + 5 Aq. 

 Heulandite 3J Al S 3 + Cal S 3 + 6 Aq. 



11. The only other zeolite that occurs in any quantity in 

 the Kilpatrick hills is harmotome. It was noticed early by 

 mineralogists under the name of cross stone, in consequence 

 of the remarkable way in which the crystals frequently cross 

 each other ; th&t shape is not common in the harmotome of 

 the Kilpatrick hills. It is usually crystallized in right rect- 

 angular prisms.* But the common modification is a terminal 

 four-sided pyramid, consisting of faces replacing the solid an- 

 gles of the base of the prism. 



The mineral called harmotome ought to be divided into 

 three distinct species, which might be distinguished by the 

 names harmotome, Philipsite, and Morvenite; the first of these 

 only occurs in our neighbourhood; but the first and third 

 are common at Strontian. The second, or Philipsite, has been 

 observed in Sicily, and in other places. The first is a com- 

 pound of silica, alumina, barytes and water; the second of 

 silica, alumina, lime and water; and in the third the propor- 

 tions of the constituents differ very much from those of the 

 second species. 



