WITH EEGAED TO WATEE SUPPLY. 
19 
report/ “ Many of the large manufacturing towns of Lancashire 
and Yorkshire are supplied with water for potable and manu- 
facturing purposes, by the storage in vast reservoirs of the upland 
drainage from these formations. Being but slightly absorbent 
they yield to the impounding rivulets and streams a large 
proportion of the actual rainfall.” Though the Millstone Grit 
may be but slightly absorbent in the localities named, yet this 
character cannot be established as a rule by which one can be 
guided. In the case of the Bristol coalfield it applies, but when 
we come to the Forest of Dean we find the reverse to be the 
case. The Millstone Grit of the West of England serves as a 
good illustration of the variability of rocks in different localities, 
especially as regards the volume of water which is capable of 
being stored in them. 
The next rocks examined were those of the Pennant Grit. 
It is necessary here to lay stress upon the definite article, as 
“the Pennant” is confined to the middle coal-measures of 
Bristol, and is also extensively developed in the Somersetshire 
and South Wales coalfields. There are, however, beds of grit 
in the lower coal-measures of Bristol, which are lithologically 
true Pennants. The Pennant Grit is a compact blue rock, 
made up of angular grains. The following is the mean analysis 
of seven samples taken from my paper on the “ Composition of 
the Pennant Grit.”^ 
Silica 
Alumina ... 
Oxide of Iron 
Lime 
Carbon ... 
Carbonic Acid 
Magnesia 
Water 
99-96 
4-34 
4-55 
1 - 29 
2 - 86 
1-33 
•05 
*58 
^ Page 40, part 2. 
2 Journal of the Chem. Society^ 1882, page 79. 
