Two fossils

We took Sheba to the dog park. The river was low and slow, the sun warm, and all played happily. My daughter found the piece of petrified wood here. There's lots of it, washing down from the mountains, enough so that (so the experts say) just about anywhere you look down by the river, there'll be a piece somewhere. Yet I've never found one, my wife has only found some once. My daughter finds something nearly every time, perhaps because she's closer to the ground, or because she's much more focused on interesting rocks than I am. I found the black thing, which I convinced myself is coal, not charcoal. Hard, dense, a bit shiny; perhaps anthracite, the oldest and most valuable of coals? They used to mine coal in the banks of the river valley here, but the mines have been closed for decades.

They are both fossils, in their own way. A fossil is anything left from an ancient living thing, but the petrified wood is the traditional kind, where the actual dead organism is replaced, bit by bit, atom by atom, with rock, leaving only the shape behind. A copy, in other words, from a fragile living thing into something more permanent. An echo of the living thing, not the living matter itself. This makes sense to me, rock is permanent, or at least long-lasting, but not living things. The coal is also fossil, but of a kind completely different. The macro structure is lost, and we can no longer tell exactly what kind of plants died and decayed in the bottom of the swamp. We cannot see their leaves and stems. All that is left is carbonaceous organic residue, the very living matter itself, the stuff which is lost and replaced by stone in petrified wood. I find this amazing. Molecular fossils, this is called, and though they are as common as the fuel you put into your gas tank, I still find it incredible to touch the (only slightly crushed and transformed) living tissues of plants which died tens of million years ago. Molecular fossils.