408 Dr. T. Thomson on the Minerals 



Gehlen, showed that the mesotype of Haiiy ought to be di- 

 vided into three species, which he distinguished by the names 

 of mesotype, needlestone, and Thomsonite. The first two of 

 these constitute the natrolite and scolezite of Fuchs and Geh- 

 len ; but the third is a new species, which Mr. Brooke first 

 described. He showed that these minerals differ in their 

 crystalline shape and in their specific gravity; Thomsonite 

 being the heaviest, and natrolite or mesotype the lightest. 



Thomsonite is a beautiful mineral, and is rather abundant 

 both in the Kilpatrick hills, and those on the south side of 

 the Clyde. When pure it is snow-white ; the crystal is a right 

 rectangular prism. The lustre is vitreous, and the specific 

 gravity is about 2'37, according to Brooke ; though I have 

 found it as low as 2'29. Before the blow-pipe it swells. It 

 contains about 13 per cent, of water, and is composed of 



There is a curious variety which occurs at Ballimony, in 

 the north of Ireland ; its colour is brown, and it is not cry- 

 stallized. Its specific gravity is only 2'29 ; but its chemical 

 analysis shows the same constitution as the Thomsonite of 

 Kilpatrick*. 



There is another variety which occurs at Port Rush, which 

 deserves attention. It constitutes small spheres about the 

 size of a pea in an amygdaloidal rock ; these spherules are 

 composed of needles radiating from the centre to the circum- 

 ference. Colour white, with a slight tinge of yellow; translu- 

 cent. Lustre vitreous. Specific gravity 2-366, and it is rather 

 softer than common Thomsonite. It froths before the blow- 

 pipe, like the other zeolites. Its composition is the same as 

 that of Thomsonite ; but its proportion of alumina is rather 

 less, and it contains 6^ per cent, of soda, and the proportion 

 of water is less. If we reckon 



Thomsonite 3 Cal S +13 Al S + 10 Aq, the Port Rush 

 mineral will be 3 Cal S + 12 Al S +N S + 8 Aq. 



My son, to whom I was indebted for the chemical analysis 

 and description of this mineral, distinguished it by the name 

 of Scoulerite, in honour of Dr. Scouler, to whose zeal and 

 abilities the mineralogy of Ireland lies under so many obliga- 

 tions. 



3. Natrolite. This mineral constitutes the mesotype of 

 Brooke. The name natrolite was first applied to a yellow 

 mammillary variety from Hb'gan, near Lake Constance, in 

 which he discovered no less than 6^ per cent, of soda. When 

 pure it is quite white, and the crystal is a right rhombic prism, 

 deviating by only 1 10' from a rectangular prism. The 

 * Analysed by Dr. R. D. Thomson. 



