Breaking the surface

The pebble is a small but perfectly integrated part of a metal factory. This factory has produced copper, silver, zinc, lead and gold (real gold, not its iron sulphide facsimile, pyrite). It is about 100 kilometres long and 60 kilometres across, by about 6 kilometres deep. It is called Wales. The metals have sustained, puzzled, frustrated, and finally abandoned many generations of Welsh miners. Many hundreds of generations, indeed, for these metals have been sought, avidly, since at least the Bronze Age, more than 3000 years ago, when shafts were dug through solid rock with little more than hand-held antler bone and rounded cobble. It is no small feat to chase the metal underground, for its path is tortuous, its presence capricious and its surroundings dangerous. The Welsh miners have been celebrated at home in literature and songs, and also in more surprising quarters, as in the Japanese filmmaker Hayao Miyazaki’s portrayal of them in Castle in the Sky (a children’s animé film, perhaps, but deeply serious at core, like everything that Miyazaki has done). So how is a country-sized metal factory created? Tiny fragments of the answer reside within the pebble. A streak of white crosses the pebble, cutting across both the strata and the tectonic cleavage surfaces. Cutting both these fabrics, it must then be younger. Such evidence of what-came-first and what-came-next is at the heart of geology, and has been so since the very beginnings of the science, since before geological time was pinned and measured by the application of atomic clocks and of fossil time-zonations. And for all today’s shiny atom-counting machines and well-stocked libraries and museums, this kind of logic is still the first thing the geologist applies when any new and unfamiliar problem comes into view. But what is it in the pebble that is younger? Peer with the hand lens, and the white streak is resolved as a mineral vein: that is, as a mass of tiny crystals that have grown within a fracture in the rock.