22 
THE POROSITY AND DENSITY OF ROCKS 
are those of the Oolites. The specimens of the Great Oolite 
were personally selected from near Bath. The average specific 
gravity is 2-52, the water absorbed by a cubic foot of rock 1’706 
of a gallon, or 142,000,000 gallons to a square mile 3 feet thick. 
The soft variety is the most absorbent. The specimens of 
Inferior Oolite were personally selected from near Cheltenham^ 
with the exception of one from near Bath. As this rock is so 
extensively used for building, and on account of the great volume 
of water which the beds contain, I have examined a number of 
representative specimens, but the variation in the porosity is so 
great that no reliable average can be given of the volume 
of water capable of being absorbed. The bed which absorbed 
the least was a hard variety of Oolite taken from below the 
Pisolite bed of Leckhampton Hill, near Cheltenham ; and the 
bed which aborbed the most was a soft vaiiety of freestone from 
the same locality, hut higher up in the series. The first of 
these gave 0‘]46 of a gallon to the cubic foot, or 13,000,000 
gallons to the square mile 3 feet thick; and the second 2-202 
gallons to the cubic foot, or 184,000,000 gallons to a square 
mile 3 feet thick. We, may, therefore, take it that the yield of 
water from the Inferior Oolite varies between those limits. 
The lieJation of Sjjecific Gravity to Porosity. — In the report 
on Selection and Decay of Stone of the Houses of Parliament in 
1839,^ it is said that the specimens of rock which had the 
greatest specific gravity absorb the least quantity of water, 
though there are individual exceptions. I cannot say that my 
observations bear out this rule, and I doubt whether any such 
rule can be laid down. 
The PieIatio7i of the Size of Grains conqwsiny a Bock to the 
Porosity. — In the tabulated results I have given a column in 
which the size of the grains composing the rocks is given. The 
object of this was to ascertain whether any relation existed 
^ Page 36. 
