Darwin's Stonehenge Notes
I have written before about Darwin's Stone at Stonehenge - https://www.sarsen.org/2014/05/darwin-stonehenge-and-worms.html where he is investigating the actions of worms in burying stones.
His manuscript notes are online:
The stone seems to be Stone 45
A further manuscript where he measures the build up of mould in the ditch is also online :