234 EEPOBT— 1882. 



becomes a matter of great importance to villages and towns in the 

 vicinity. 



The only specimen of Triassic sandstone which I have had an oppor- 

 tunity of examining is that of the Banter, from Heidelberg, Germany. 

 The specific gravity was 2'55, and the volume of water absoi-bed 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 absorption to be 0"733 of a gallon of water to the cubic foot of rock. 



The Calcareous Rocks. — Coming to the calcareous rocks which I have 

 examined, I have classed among them the Magnesian Conglomerate, 

 which I take first. The average specific gravity is 2 '73, and the volume 

 of water absorbed by a cubic foot of rock varies between 0"082 and 

 0-368 of a gallon, or between 6,000,000 and 30,000,000 gallons to a square 

 mile 3 feet thick. This rock is extensively used for building purposes, 

 and a knowledge as to the porosity of the various beds is therefore 

 important in this respect as well as for water supply. It would appear 

 from the specimens which I have examined, that the finer the conglo- 

 merate the more water there is absorbed. 



Of the Magnesian Limestone, two specimens were examined, both 

 selected from the same locality. The average mean specific gravity is 

 2'77, the water absorbed by a cubic foot of the rock 1'031 gallons, and by 

 a square mile 3 feet thick, 86,000,000 gallons. 



The Magnesian Limestone is much more porous than the Carboniferous 

 Limestone, which I take next. All the specimens were obtained per- 

 sonally, the first three from the lower shales. These gave a specific 

 gravity of 2' 71, with a porosity of 0'028 of a gallon of water to a cubic 

 foot of rock, which is equal to 2,000,000 gallons to a square mile. The 

 specific gravity of the specimens representing the limestone gave an 

 average of 2'70. The volume of water absorbed by a cubic foot was '043 

 of a gallon, or 3^ million gallons to a square mile 3 feet thick. From 

 these comparisons it would seem that the lower shales are least porous. 

 Considering the large volume of water which these rocks supply, their 

 not being porous will seem contradictory ; the fact is, however, that the 

 water finds its way through joints and fissiires, dissolving away the lime- 

 stone, and sometimes forming subterranean reservoirs. 



We next come to rocks which are very pervious ; some of them, so far 

 as their absorption of water is concerned, may be compared to an ordinary 

 sponge. The rocks to which 1 allude 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 absorbed the 

 most was a soft variety of freestone from the same locality, but higher 

 ' Fourth Report Underground Water Committee, page 16. 



