WITH EEGARD TO WATEK SUPPLY. 
17 
In some of the specimens of this grit, microscopically 
examined, the grains of silica appear to cement themselves 
together, and so closely, that it is difficult to distinguish their 
outline. The grains are, for the most part, sub-angular, and 
are either colourless or have a slight pink tinge imparted hy 
oxide of iron. The specific gravity averages 2 60, and the 
amount of water absorbed by a cubic foot of the rock is 0*080 of 
a gallon, equal to 6,000,000 gallons to a square mile 3 feet thick. 
Two typical specimens of Millstone Grit were selected from 
Pertyrch, South Wales ; one a coarse variety and the other with 
grains averaging about 0*010 of an inch in diameter. There is 
a slight lithological difference when compared with the same 
formation around Bristol, and chemically it is a little more 
argillaceous, as shown hy the following an 
Silica 
... 96*63 
Alumina 
... 1*15 
Oxide of Iron 
*70 
Lime 
*55 
Carbonic Acid 
*20 
Carbon 
*30 
Moisture 
•10 
Alkalies not estimated 
99*63 
alj^sis 
The specific gravity may he taken at 2*57, the volume of 
water capable of being absorbed hy a cubic foot of the rock 0*290 
of a gallon, or 23,000,000 gallons to a square mile 3 feet thick. 
The specimens of Millstone Grit from Sheffield resemble 
lithologically those of South Wales, though a person acquainted 
with the formations in the two districts would probably be able 
to distinguish between them. The grains composing the grit in 
the neighbourhood of Sheffield, according to Dr. Sorby, “ are, on 
the whole, extremely angular.”^ The specific gravity averages 
I Quarterly Journal Geol. Society, 188, page 64. 
0 
