416 Dr. T. Thomson on the Minerals 



It is white, has a fibrous texture, and the fibres are in tufts 

 diverging from a centre. Lustre silky, translucent on the 

 edges, softer than calcareous spar. Specific gravity 2-850. 



It is composed of silica, lime and soda, with a little mag- 

 nesia. If we include the magnesia along with the soda it will 

 be composed of 



3 atoms bisilicate of lime. 

 1 atom tersilicate of soda. 

 3 Cal S 2 -f N S 3 * 



3. Prasolite is a name by which I have distinguished a 

 mineral which occurs in the Kilpatrick hills, constituting a 

 vein about an inch in thickness. It was brought me by one 

 of my students about two years ago, who wanted to know 

 what mineral it was. From its appearance I pronounced it 

 to be common sulphate of lime. But happening to examine 

 it a little more closely, I found that I had been mistaken in 

 my opinion, and that it, in fact, constituted a new mineral 

 species, allied to the zeolites ; since it contained no less than 

 1 8 per cent, of water. 



Its colour is leek-green, and it is as soft as Venetian talc, not 

 being capable of scratching selenite. The specific gravity is 

 2'31 1 . It is composed of fibres very loosely cohering together, 

 since it may be crumbled to powder between the fingers. 



Its constituents, besides water, are silica, magnesia, per- 

 oxide of iron and alumina, and probably soda. Its consti- 

 tution may be represented thus : 



9Mg Sii + 4fSii + 3AlSH+18 Aq, or 



3 (| Mg + i Al) SH + f Sii + 4i Aq; 



Fluor spar is found sparingly in cubic crystals in the trap 

 rocks near Gourock. 



Prehnite is so exceedingly abundant, that at one time a 

 whole cartful of it was brought to Glasgow. 



I need not notice augite and amphibole, felspar and albite, 

 which constitute the common constituents of the greenstone 

 rocks so abundant in this part of Scotland. But it is right 

 to notice labradorite, which constitutes one of the constituents 

 of a peculiar variety of greenstone which may be seen in a 

 state of perfection in Campsie glen, and which constitutes 

 the principal rock in the hills south of Paisley. In these 

 rocks it is very conspicuous, because they have been altered 

 by the action of the weather, and the labradorite has become 

 white, while the hornblende, the other constituent, retains its 

 dark colour. 



* Analysed by myself and Dr. R. D. Thomson. Records of General 

 Science, i. 220. 



