The history of ideas on extensional tectonics is somewhat neglected in the general historiography of geology. This is because the occurrence of widespread extension on our planet was recognised late, although normal dip-slip faults are by far the most common of fault types and this had been noticed since the early days of mining. A review of the geological literature shows, however, that many geologists and miners did think of widespread extension on earth very early on. Their ideas did not have the same impact as ideas on shortening structures, because the latter are visually so much more spectacular in Europe, where the science of geology was born. Observations thus do govern the spread of ideas in the sciences, contrary to what many in the social disciplines claim.