All around Ireland this morning, geologists are trimming their beards and polishing their hammers, as they prepare to bring Irish Geology Week to a fitting climax with suitably restrained and decorous zeal. In Dublin the highlight is a geological exhibition, modestly entitled "Down to Earth", at the headquarters of the Geological Survey of Ireland in Haddington Road.
It contains a myriad of rock samples, some of quite spectacular beauty, and is also a vast reservoir of information about Ireland's geological make-up and the ways in which it has been discovered and explored.
The exhibition opens today from 1 p.m. to 5 p.m., and a team of geologists will be on hand to answer questions. Indeed if you care to bring a rock sample of your own, they will more than likely tell you where you found it, where it came from, and, give or take a million years or two, how old it is.
But geologists have a funny way of reckoning time: instead of hours and minutes, years or even centuries, they think in terms of "periods" and "eras", and tie down a significant event even more precisely to an "age" or "epoch".
When the system was first devised, geologic time was divided simply into four "periods", which with indisputable logic were called one, two, three and four. Only the last two of these, however, are still used: number three - or the Tertiary Period - lasted from 65 million to two million years ago; the fourth, or Quaternary Period, was that from two million years ago until the present day.
Fresh discoveries, however, brought a new complexity. Nowadays eternity is divided by geologists into four "eras": starting from the beginning of the world they are the Precambrian, the Paleozoic, the Mesozoic, and Cenozoic. Each of these eras is divided into "periods"; the two most recent periods, which together form the current Cenozoic Era are the already mentioned Quaternary and Tertiary. When dating must be even more specific, each period is divided into a number of "epochs" - the current Quaternary Period, for example, consisting of two epochs, the Holocene and Pleistocene. The Tertiary period, on the other hand, is subdivided into five, the Pliocene, Miocene, Oligocene, Eocene and Paleocene epochs.
One way of making sense of this unpronounceable tangle of chronological complexity is to relate it to the conventional calendar. If you think of an "era" as being a year, then a "period" is analogous to a month, an "epoch" to a week, and an "age" to a single day.