Omar, G.I., T.C. Onstott, and J. Hoek. 2003. “The Origin of Deep Subsurface Microbial Communities in the Witwatersrand Basin, South Africa As Deduced from Apatite Fission Track Analyses”. Geofluids 3: 69-80. doi:10.1046/j.1468-8123.2003.00050.x.