Porosity of the Bocks of the Karroo System. 
171 
tests were made upon the sandstones, which, as a rule, are fine to 
medium-grained in texture and felspathic in character. 
The basal member of the Karroo System in the Cape Province is the 
Dwyka conglomerate of glacial origin, a rather compact and somewhat 
impervious material ; the Ecca shales which follow it are fine-grained with 
an average porosity not exceeding 2 % very probably, except in the 
extreme south-west, where a sandy phase has been developed locally. 
These two formations crop out along the margin of the Karroo " basin," 
as shown on a geological map of the country. The Beaufort Series 
overlying the Ecca is characterized by the presence of sandstones and 
is the most important member of the Karroo System in South Africa 
in this investigation, inasmuch as it covers an area very much greater 
than that constituted by all the other series put together. Determinations 
of Beaufort rocks consequently preponderate in the table attached. 
Even in the Beaufort Series the sandstones, which never possess more 
than a medium grain, do not predominate, for probably more than half of 
the strata consist of rather close-grained thinly bedded green and blue 
flaggy sandstones, hard nodular mudstones commonly somewhat gritty, 
and soft green, red, and purple clayey rocks, these bright-coloured layers 
being best developed in the upper portion of the formation. 
The Beaufort Series has, partly upon lithological grounds, been sub- 
divided into three groups, and in the table appended this three-fold 
classification has been developed, subject of course to such errors as 
may be brought in by uncertainty as to the exact geological horizon 
in any particular case. It has already been pointed out by Dr. C. F. 
Juritz''' that underground waters of distinctive chemical character are 
furnished by each of these sub-divisions, and it can now be shown 
that the rocks of the three groups exhibit as well a variation in 
their capacity for absorbing water. Thus, for example, the sand- 
stones of the Lower Beaufort Beds have a mean porosity of 2*9 % (the 
average of 13 determinations, excluding No. 21, which was an un- 
commonly porous type of rock yielding a strong supply of water). By 
still further analysis it is found that sandstones coming from the 
higher horizons of this sub-division are more porous than those of the 
lower zones, the actual figures being 4-9 % (4 determinations) as against 
2-0 % (9 determinations). 
The Middle Beaufort sandstones are not so close-grained and furnish 
a value of 5-2 % (8 determinations), while the Upper Beaufort sandstones 
stand a trifle higher, 5'5 % (14 determinations). The finer-graiaed rocks 
of the Upper Beaufort series are much more permeable than those of the 
Lower ; the figures being 4-9 % (5) and 1'4 % (4) respectively. 
* C. F. Juritz. Agric. Journ. of C.G.H., vols, xxxii. and xxxiii., 1908. Presidential 
Address to Cape Chemical Society, 1908. 
