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ON THE POEOSITY OF THE EOCKS OF THE KAEROO 
SYSTEM IN SOUTH AFEICA. 
By Alex. L. du Toit, D.Sc, F.G.S. 
(Published with the permission of the Hon. the Minister for Mines and 
Industries.) 
(Eeceived April 7, 1914. Eead April 15, 1914.) 
In a paper read last year before the South African Society of Civil 
Engineers the author stated that the strata in South Africa taken as a 
whole possessed a rather low degree of porosity, and that this was 
especially applicable to the rocks of the Karroo System, covering, as they 
do, fully one-half of the Union of South Africa. 
It was pointed out moreover, that, as the result of weathering, any 
estimate of the pore-space of a rock as based upon specimens taken from 
the outcrop of the stratum, as is generally the case, is in excess of its true 
value as found when dealing with the fresh and unaltered material. 
For these and for other reasons the Karroo region was not anticipated 
to be one capable of furnishing strong artesian supplies as has not un- 
commonly been thought. 
Since, however, these statements were subjected to criticism, a large 
number of determinations were made upon rocks from the Karroo System 
of the Cape Province, a number of figures obtained in the Geological 
Survey Laboratory, Pretoria, upon certain Karroo sandstones of the 
Transvaal f being also included in the list. 
The results, tabulated below, bear out in the fullest manner the 
author's contentions ; time did not permit of an exhaustive investigation 
of this rather wide and important subject, for several interesting dependent 
problems emerged in the course of the tests for fuller inquiry and treatment 
at some future period. 
The specimens experimented upon consisted for the most part of 
* Min. of Proc. S.A. Soc. Civil Engineers, vol. xi., p. 8. Cape Town, 1913. 
f Annual Report Geological Survey of S. Africa for 1910, p. 105, Pretoria. 
