64 
from Dr. Hassall’s 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 he 
some small lakes from which the springs are supposed to 
derive their principal supply of water, which, after perco- 
lating through the above beds and dissolving large quanti- 
ties of their constituents, flnds 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. HassalFs analysis 
and some of his remarks relative to the geological position 
of the springs to the pamphlet of Dr. Hayward previously 
mentioned. 
