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Transactions of the JRoyal Society of South Africa. 
The Molteno sandstones of the Stormberg Series being coarser in grain 
and less compacted give a higher figure 10-9 % (2), but more determinations 
are required to be made upon this formation and upon the finer-grained 
Cave Sandstone also. 
It must be pointed out that, for the purpose of obtaining these figures, 
all samples taken from within 25 feet of the surface of the ground were 
rigidly excluded. 
Proceeding from the Cape into the Transvaal, the Karroo System 
becomes thinner, this being partly due to the disappearance of certain 
zones represented in the Cape, namely the Ecca shales, and apparently 
the Molteno Series as well. At the same time the whole succession above 
the Dwyka becomes more arenaceous in character, and is now divisible 
into the Coal Measure grits and sandstones (probably representing some 
portion of the Beaufort Series), succeeded by the finer-grained Bushveld 
sandstone (apparently the equivalent of the upper part of the Stormberg 
Series). 
The majority of the figures given for this section were determined 
upon samples of building stone obtained from quarries, and obviously 
the results, which are high, are not strictly comparable with those from 
the Cape, for the average porosity of three specimens of sandstone-core 
from the borings on the New Eand along the Wilge Eiver in the Orange 
Free State is but 10*1 %. The Flatpan and Steenpan stone from the 
quarries near Coalbrook, Orange Free State, averages 12-1 % (5 determi- 
nations), but the Vereeniging stone is much more porous, a specimen of 
medium-grained sandstone giving 18"3 %, while a very fine-grained white 
sandstone was remarkable in attaining the high value of 32-8 %. 
The Coal Measure grits range from 8 % to 18 %, but the Bushveld 
sandstone is less porous with values extending from 7 % to 14 %, the 
red variety possessing a slight advantage in pore-space over the white. 
The porosity of the Transvaal phase of the Karroo System, composed 
almost throughout of such arenaceous deposits, is therefore much higher 
than that of the Cape, but, since no bore-hole cores of the former have yet 
been experimented upon, the water-bearing capacities of the two phases 
cannot satisfactorily be compared at present. 
Eelation of Porosity to Specific Gravity. 
Since the more compact rocks possess a higher density than the more 
open-grained ones, it follows that the value of the porosity of a sample 
must approximately be inversely proportional to its specific gravity. In 
the following figure these two quantities have been plotted in the case of 
90 rocks, and it will be seen that the porosities fall within well-defined 
limits, showing that, in dealing with samples having a given density, the 
