ONE OP THE TREFRIW MINERAL WATERS. 361 



remarkable for the large quantity of sulphate of alumina 

 and silicic acid which are dissolved in it, while the amounts 

 of phosphoric and nitric acids, though only small, are 

 rather large compared with what is found in most other 

 mineral waters; on the other hand the proportion of 

 chlorine is only small. 



The other Trefriw mineral spring was not analyzed ; but, 

 from Dr. HassalFs analysis of the two waters, it appears to 

 contain less iron and alumina, but a larger quantity of 

 alkalies and alkaline earths than the one which is the sub- 

 ject of this memoir. 



With regard to the geological position of Trefriw, and 

 the source of the mineral impregnation of the springs, it 

 may be observed that the mountains at the base of which 

 the wells are situated consist chiefly of beds of limestone, 

 ironstone, alum-slate, and iron-pyrites, together with vary- 

 ing proportions of silicates, very much fractured and dis- 

 located, forming the northern extremity of the Bala or 

 Caradoc beds. Up in the mountains, and on these beds, 

 lie some small lakes, from which the springs are supposed 

 to derive their principal supply of water, which, after 

 percolating through the above beds and dissolving large 

 quantities of their constituents, finds its exit near the base 

 of the mountain Alt cae Coch, where it issues from the 

 slate bed (Black band), and between it and the ironstone. 

 From the above data the composition of the water is easily 

 accounted for. There are several pyrites-mines in the 

 vicinity, one of which is situated just over the springs, 

 but much further up the mountain-side. 



The author has been indebted, for Dr. HassalPs analysis 

 and some of his remarks relative to the geological position 

 of the springs, to the pamphlet of Dr. Hay ward previously 

 mentioned. 



SER. III. vol. v. 2 B 



