18 
THE POROSITY AND DENSITY OF ROCKS 
2*59, and the volume of water absorbed per cubic foot of rock is 
0-504 of a gallon, equal to 49,000,000 gallons to a square mile 
3 feet thick. 
'J wo samples of Millstone Grit were collected from the Forest 
of Dean coalfield ; one from the southern outcrop and the other 
from the northern. We here get a very different lithological 
character when compared with the same formation at Bristol 
and in South Wales, and a very much larger volume of water is 
absorbed. The rock resembles a Trias sandstone more than the 
Millstone Grit, but in chemical composition there is practically 
little difference, as is shown by the following analysis of a 
specimen from Drybrook : — 
Silica 
... 
... 98-06 
Alumina 
•30 
Oxide of Iron ... 
•50 
Lime 
•33 
Carbon ... 
•20 
Carbonic Acid ... 
•30 
Alkalies 
... Trace 
99-69 
The specific gravity of the Forest of Dean Millstone Grit 
averages 9-63, the volume of water absorbed by a cubic foot of 
the rock is 0-854 of a gallon, or 71,000,000 gallons to a square 
mile 3 feet thick. Though I have given the average volume of 
water absorbed, it will be seen, on reference to the tabulated list, 
that the results obtained from the two specimens examined 
differ considerably. In the case of the one absorbing the 
greatest quantity of water, the grains composing it were but 
slightly cohesive, but in the other case the rock was of a more 
compact character. 
Referring to the upland surface water from the Millstone 
Grit and the non-calcareous portions of the coal measures, the 
Royal Commissioners on Rivers Pollution say in their sixth 
