174 Transactions of the Boyal Society of South Africa. 
the cementing matter. Beyond this stage thoroughly decomposed rocks 
would have to be included, and after that the soils produced by them ; the 
consideration of these is outside the scope of this paper, however. 
Effect of Weathering upon Porosity. 
Most of the Karroo sandstones (those of the Molteno Beds excepted), 
and not a few mudstones, possess a cement of a calcareous nature ; this 
forms generally not more than 2 % of the weight of the rock, but 
occasionally the proportion rises to 4 % or even 5 %. At the surface and 
down to a moderate depth this binding material tends to be removed 
by water containing dissolved carbonic acid ; usually the calcareous 
matter is ultimately conveyed by the underground drainage into the 
rivers, but sometimes under the action of capillarity it is brought to 
the surface and there forms the white calcareous tufa, so common in 
the northern Karroo and the Orange Free State. 
Again, nearly all the sandstones contain a moderate amount of felspar, 
which in the process of weathering becomes converted into a porous mass 
of kaolin ; at the very outcrop also the finely divided kaolin "dust " will 
tend to be removed mechanically. Other minerals, such as sericite, mica, 
and chlorite, together with substances the exact mineralogical nature of 
which is uncertain, are also present as a ground-mass, and some of these 
must undergo alteration with partial or complete removal under the 
chemical action of ground-water. 
An attempt to determine the proportion of calcareous cement by 
soaking specimens in a weak solution of citric acid did not prove 
satisfactory, so recourse was had to a treatment with dilute hydrochloric 
acid, by which, however, certain silicates of alumina and to a lesser 
extent of magnesia were broken up and removed as well. It is believed, 
for reasons that will become self-evident further on, that in spite of the 
severity of this treatment the results are quite comparable with those 
produced under the action of the weather ; that the processes of the latter 
sometimes results in the solution of silicic acid is indicated by the 
extremely incoherent nature of some outcrop specimens of sandstone 
and by the secondary silicification which appears now and again on 
rock surfaces or in certain superficial deposits in South Africa. 
As an example, five specimens of sandstone lost on an average 
2-0 % of their weight by soaking in acid, this figure corresponding to 
an increase in pore-space of about the same amount ; the actual increases 
in porosity ranged from about y\th to fths of those in the untreated 
rocks, while the average increase was nearly ^rd. 
In the instances under discussion the percentage increase in porosity 
due to the removal of cementing matter ranged between 2 and 14 times 
