B. E. Lock & J. A. Robey write: We were interested to read the paper by Svensen et al. (2006) on South African hydrothermal vents within the Karoo Basin, particularly concerning two vent complexes known as Witkop II and Witkop III. One of us (B.E.L.) had barely initiated a study of the Stormberg diatremes when he left South Africa to accept a faculty position in Louisiana in 1977, and the project was not continued. However, the Svensen paper reminded us of features we had seen in the field at the Mackay's Kop diatreme, near Jamestown, in the mid-1970s. This...

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