16 
THE POROSITY AND DENSITY OF ROCKS 
exposed. The whole of the results obtained are given in the 
annexed table. 
The Arenaceous Rocks . — The oldest rocks which I have 
examined are those of the Old Red, and all the specimens were 
selected personally. Taking first the Old Red Sandstone we get 
an average specific gravity of 9*61, but the specimens from near 
Bristol have a decidedly lower specific gravity compared with 
others which are given. Excluding the flags from Caithness, 
the volume of water absorbed by a cubic foot of the rock is 
0*707 of a gallon, or 59,000,000 gallons to the square mile 
3 feet thick. The specimen of flags from Caithness absorbed 
much less water, and I found the same thing to apply to Old 
Red flagstones from other districts. The conglomerate beds are 
more absorbent than the sandstones : the average specific gravity 
is 9*58 and the volume of water absorbed by a cubic foot 0*805 
of a gallon, or 67,000,000 gallons to a square mile 3 feet thick. 
I next take the Millstone Grit. The specimens from Bristol, 
South Wales, and the Forest of Dean were selected personally, 
and for those from Sheffield I am indebted to the kindness of 
Dr. Sorby, F.R.S. The Millstone Grit which underlies the 
Bristol coalfield is som.ething like 1,000 feet thiclR; the chemical 
composition, the mean of five analyses, is as follows : — 
Silica 
... 97*80 
Alumina 
•47 
Oxide of Iron ... 
•80 
Lime 
•44 
Carbon 
•17 
Carbonic Acid ... 
•39 
Moisture 
•22 
100-29 
Proc. Bristol Naturalists' Society, 1875-6, page 336. 
