Tidal Phenomena at Inland Boreholes near Cradock. 63 
were successful in yielding over 1,000 galls, of water a day. When 
rocks of pre-Karroo age were bored the percentage of success was 
much smaller. 
The Eeports further show that of the wells bored in the Karroo System 
about 25 per cent, yielded " flowing " or artesian water, while about 50 
per cent, yielded subartesian water rising to within pumpable reach of the 
surface. 
In nearly all cases in which water was struck the water rose in the 
borehole some distance above the stratum in which the water was first 
encountered. 
During the last eight years a large number of holes have been bored in 
addition to these Government drillings. The Eeport of 1909 already 
quoted refers to the existence at that date of 102 private boring con- 
tractors in the Colony, and the Chief Inspector of Water-Boring in his 
Eeport for 1907 states that these boring contractors were putting down 
boreholes at the rate of 1,400 a year. As a rule these contractors bore 
only in the Karroo System or in localities where past experience has 
shown a considerable prospect of success. I estimate that altogether at 
the present day there must be over 10,000 successful boreholes iji Cape 
Colony. 
I have at various times visited a large number of these boreholes both 
during their construction and afterwards. It has become obvious to me 
that the vast majority of them merely tap shallow supplies dependent on 
the local rainfall, and that they are nearly all liable to give a diminished 
output or in some cases to dry up altogether after local droughts. 
The temperature of most of these waters is approximately the same as 
the probable rock temperature within a few hundred feet of the surface. 
For purposes of irrigation the quantity of water yielded is, with few 
exceptions, insignificant, but for the purpose of watering stock and for 
domestic use these wells are of the utmost value to the country. 
Practically the only attempt at a general theory of the artesian 
pressure, movement, and position of underground water in the Karroo 
System that has been hitherto recognised is what might be termed the 
" dolerite intrusion " theory. 
This theory is fully explained in its various applications by Mr. H. P. 
Saunders in a Government Blue Book published in 1897 and entitled 
" Underground Water-supply of the Colony and the Cape of Good Hope." 
The substance of this theory is that the water enters the more pervious 
layers of Karroo sediments at their outcrops on elevated ground and per- 
colates along these layers through pores and cracks to lower levels until 
stopped and dammed back by one of the numerous impervious dolerite 
intrusions. That underground dams of waterlogged rock are thus produced 
in which the water, under hydrostatic pressure, is held down by super- 
