170 Transactions of the Royal Society of South Africa. 
sections of core from diamond drill borings put down by the Public Works 
Department under the late Cape Government, samples duly labelled 
and accompanied by full records of the strata passed through being 
preserved in the Geological Survey Office, Cape Town. The cylindrical 
specimens, varying in weight from about 100 to 300 grams, were dried, 
weighed, and soaked in water until no further absorption took place. 
The smaller-sized samples were preferred because of the greater rapidity 
with which they absorbed water ; this was especially so in the case of 
very soft shales and mudstones, the cylinders of which frequently will 
not stand soaking for more than a few days' time without breaking up. 
Indeed, the only accurate procedure with the soft red and purple mud- 
stones would be to work in the field upon wet cores just extracted from 
the borehole. Since such friable argillaceous rocks possess only a low 
degree of porosity, few determinations were made upon them. 
With certain exceptions, which were instituted for purposes of com- 
parison, all the samples were from strata cut below a depth of 25 feet 
from the surface, while the majority came from between 50 and 150 feet ; 
in this way the values obtained must approach more closely to those 
under the conditions actually existing in boreholes and wells. The 
increased value of the porosity dependent upon surface alterations will 
be discussed later on. 
Care was taken in the selection of the core samples that these did not 
come from layers directly over or underlain by dolerite, sheets of which 
are prevalent throughout the Karroo except on its extreme southern 
margin and in the south of the Transvaal. Indeed, not uncommonly the 
soft shales and mudstones have been converted by the igneous rock 
into black flinty hornstone or lydianite, the pore-space of which is 
practically nil. 
In the table two columns of figures are given, the first of which 
represents the percentage weight of water ^ absorbed, or the Batio of 
Absorption,''' the second being the percentage of the volume of the rock 
which can be occupied by the water imbibed; this is the percentage of 
pore-space or the Porosity. The porosity, as a matter of fact, is equal to 
the ratio of absorption multiplied by the specific gravity of the rock in 
question. 
Relation of Pobosity to Geological Hobizon. 
The sandstones by reason of their greater pore-space naturally form 
the most suitable criteria in the comparison of the relative porosity of 
successive geological horizons, and for this reason the majority of the 
* E. R. Buckley. Wisconsin Geological and Natural History Survey, Bull, 4, 
Economic Series 2, p. 68, 1898. 
