20 
THE POROSITY AND DENSITY OF ROCKS 
The specific gravity of the Pennant averages ^2-67, the \yater 
absorbed by a cubic foot of the rock 0T50 of a gallon, and by a 
square mile 3 feet thick 12,000,000 gallons. Speaking of deep 
^vells in the Coal Measures, the Rivers Pollution Commissioners 
say The proportion of mineral impurity present in the deep 
well-water is always large, but varies within wide limits. The 
water was found to contain, “ as a rule, larger proportions of 
organic elements (organic carbon and organic nitrogen) than are 
met with in similar waters obtained from other strata,” the 
average proportion being 0'153 parts per 100,000 parts, or *107 
grains per gallon. I quite endorse what the Commissioners say 
in respect to water from the coal measures generally, but where 
we have a great thickness of rock, as in the case of the Pennant 
around Bristol and Swansea, I think there may be an exception 
to wljat has been stated. Some years ago, it was proposed to 
supply a portion of Bristol with water from the Frampton 
Cottrell iron mines, which are in the Pennant, and the analyses 
made of the water showed it to be of good qualitj'’. Considering 
the quantity of water which this rock is capable of storing, the 
quality becomes a matter of great importance to villages and 
towns in the vicinity. 
The only specimen of Triassic sandstone which I have had 
an opportunity of examining is that of the Bunter, from Ideidel- 
bei’g, Gcrniaiiy. The specific gravity was 2-55, and the volume 
of water absorbed by a cubic foot of the rock was 0-838 of a 
gallon, equal to 70,000,000 gallons to a square mile 3 feet thick. 
Mr. I. Roberts. F.G S.,^ has made observations on the porosity 
of the Bunter from the Pebble Bed of Everton, and found the 
absoi-ption to be 0-733 of a gallon of water to the cubic foot of 
rock. 
The Calcareous Rocls. — Coming to the calcareous I'ocks 
^ Sixth Report, page 91. 
2 Fourth Report Underground Water Committee, page 16. 
