Porosity of the Bocks of the Karroo System. 
175 
the percentage decrease in weight of the rock during treatment. Some of 
the material extracted by the dilute hydrochloric acid (and therefore 
present in the form of chlorides of the bases along with silicic acid) was 
evaporated to dryness, and the residue, when examined chemically, was 
found to have the following composition (the bases being reckoned as 
oxides) : silica, 33 % ; oxides of alumina and iron, 24 % ; lime, 26 % ; and 
magnesia, 10-8 %. 
Comparing the figures obtained for the porosities of samples taken from 
the surface, or immediately below the surface, with those of specimens 
from greater depths, the average values for the Beaufort Series work out 
as follows : — 
Lower Beaufort, weathered, 5-1 % (3 determinations) 
Lower Beaufort, unweathered, 2*9 % (13 determinations) 
Middle Beaufort, weathered, 5"6 % (2 determinations) 
Middle Beaufort, unweathered, 5-2 % (8 determinations) 
Upper Beaufort, weathered, 9*7 % (4 determinations) 
Upper Beaufort, unweathered, 5*5 % (14 determinations) 
It is thus seen that substantial increases in porosity have been brought 
about within the surface zone ; incidentally this fact should form a very 
strong argument in favour of deep quarrying for the supply of durable 
building stone, a matter which has consistently been ignored throughout 
South Africa. 
Eelation of Porosity to Water Storage. 
When the figures for the Karroo rocks are compared with those for 
sandstones in other countries,"^' it is found that the South African rocks 
as a whole possess quite a low degree of porosity, the Lower Beaufort 
Beds being most marked in this respect ; only the Transvaal Coal 
Measures can be regarded as being moderately porous. 
The actual capacity of a stratum for storing water is fortunately, 
however, not the same thing as its porosity, for, in addition to the 
capillary passages between the constituent grains, there are numerous 
minute cracks and joints which traverse the rock in various directions, 
most frequently in planes at right angles to the stratification. 
Such cracks and joints are most numerous and widest as a rule at 
the surface of the ground, and they become of less importance with depth ; 
generally below the 400-foot level the capacity of the rock does not differ 
much from its porosity. Above that level the capacity is much greater, 
and in the uppermost zone it may greatly exceed the value found for the 
pore space, especially if the porosity of the rock be low. 
* E. E. Buckley, I.e., p. 400. 0. Lueger. Die Wasserversorgung der Stadte, pp. 217-19, 
Stuttgart. 
