182 TI. G. BONNEY [sEpr. 1899 
Discharges of lava occurred during the Karoo period and probably 
afterwards (for both the pipes and the surrounding sedimentary rocks 
are pierced by dykes), while the marked changes in the matrix of the 
blue ground (what has been one of the great difficulties in determining 
its real nature) suggest that for a long time it was acted upon by 
water at a high temperature. Thus the volcanoes did not go beyond 
the solfataric stage. They occur over a rather extensive district 
and are fairly numerous—comparable, in fact, with the volcanic necks 
of Fifeshire. 
The diamantiferous boulders obviously have no connection with any 
existing alluvia. Probably they have come from a conglomerate at the 
base of the sedimentary series, resting directly on the crystalline floor, 
Thus far we have no means of determining what the age of the latter 
may be, but the Dwyka conglomerate of South African geologists — 
generally assigned to the Permian system — very probably extends 
beneath the Karoo beds of the diamantiferous region, and may 
repose on the crystalline floor. On that point, however, we must 
await further evidence; suffice it to say that the genesis of the 
diamond in South Africa was not a phenomenon of Mesozoic or 
later times, but must be yet more ancient. 
23 DENNING Roap, 
Lonpon, N.W. 
