WITH REGARD TO WATER SUPPLY. 
21 
which I have examined, I have classed among them the Mag- 
nesian 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 betw-een 
6.000. 000 and 30,000,000 gallons to a square mile 3 feet thick. 
This rock is extensively used for building purposes, aiid a know- 
ledge as to the porosity of the various beds is therefore important 
in this respect as w-ell as for water supply. It w-ould appear 
from the specimens w’hich I have examined, that the finer the 
-conglomerate the more w-ater there is absorbed. 
Of the Magnesian Limestone, two specimens w'ere 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 personally, 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, wLich 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 w^ater absorbed by a cubic foot w^as *043 of a 
gallon, or 3^ million gallons to a square mile 3 feet thick. From 
these comparisons it would seem that the low*er shales are least 
porous. Considering the large volume of water w^hich these 
rocks supply, their not being porous will seem contradictory ; the 
fact is, however, that the water finds its w-ay through joints and 
fissures, dissolving away the limestone, and sometimes forming 
subterannean reservoirs. 
We next come to rocks wdiich 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 I allude 
