Nineteenth century geologists recognized that rocks formed slowly as mountains eroded and sediments settled on the ocean floor.But they could not say just how long such processes had taken, and thus how old their fossils were.Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes.One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.
But Kelvin didn't, and couldn't, know that radioactive atoms such as uranium were breaking down and keeping the planet warmer than it would be otherwise.
This combined X-ray elemental map shows Mg (red), Ca (green) and Al (blue) of the CR carbonaceous chondrite PCA 91082. Rocks like these preserve a record of the processes and timing of events in the solar nebula. This drawing depicts some of the processes that might have operated in the nebular disk surrounding the young Sun.