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Transactions of the Boyal Society of South Africa. 
incumbent shale or other impervious strata and held back horizontally by 
impervious dolerite dykes. 
I have in the course of my v^anderings seen much to support the 
opinion that this theory embodies a considerable amount of truth, but 
I have also come to the conclusion that it is far from containing the whole 
truth. Its application is confined to the obviously shallow local supplies 
of water, and it fails to meet the case of the few hot springs and hot- water 
wells that also occur in the Karroo System. 
My observations have led me to hold the view that the waters of the 
Karroo System can be sharply divided into two great classes. On the one 
hand, those waters that are superficial in position, having variable tempera- 
tures of from 66 to 70° F. as in the vast majority of the boreholes and 
natural springs of the country ; and on the other hand, those waters that 
are characterised by high temperatures of from about 75 to nearly 100° F., 
by the presence of sulphuretted hydrogen and methane gas, and generally 
also by the presence of a curious bacterium which flourishes in sulphurous 
waters. This deep-seated water is best known at a number of points 
where it emerges through natural springs, as at Aliwal North and Cradock. 
As nearly all the boreholes extend to shallow depths, it has happened 
(as was to be expected) that very few have succeeded in tapping this deep- 
seated sulphurous water. 
The happy accident of a shallow borehole having intersected a fissure 
leading down to great depths or a porous stratum fed by such fissures 
accounts for the few exceptions. Irregular fissures leading down to great 
depths also supply the only feasible explanation of the occurrence of such 
natural springs of thermal water as are seen at Aliwal North and Cradock. 
When this contrast of the two classes of Karroo water had struck me, 
I began to feel that a careful study of the sulphurous occurrences would 
be most likely to lead to a better theory of the underground water of the 
Karroo, and my attention has since then been largely concentrated on that 
class, though intermittently, as the time at my disposal for such work was 
confined to infrequent long academic vacations, and each visit to the 
interior involved my spending three or four days in railway trains. 
The position of the group of boreholes which I have studied in mosfc 
detail is on a farm formerly called " Driefontein," but now known as 
" Tarka Bridge." It lies about 15 miles to the south-east of the town of 
Cradock, and on it the junction of the Tarka Eiver and Fish Eiver occurs. 
The arable land forming the greater portion of the farm lies between 2,700 
and 2,800 feet above sea-level, while the mountainous portions on the south- 
east and east rise much higher — probably exceeding 4,000 feet in parts. 
The geological structure of the farm is very simple. A series of shales 
and sandstones with a very slight northward dip and a few intrusive 
dykes and sills of dolerite are the principal features. The only fossil 
