Having finally got hold of Mr Cope's book (free from a friend in a bookshop as some pages were loose!) I recently road tested it with a visit to Arthur's Stone in the Brecons. I was surprised to read this entry "Chris Barber has noted that an unexplainable type of ancient mortar is to be found in a small crevice on the West side of the huge capstone. Geological analysis in May 1957 found the cement to contain large amounts of coal dust".

Anyone know if there is such a thing as Neolithic cement? I can't find much about it on the net although I did find this, relating to the cleaning of some Neolithic statues, that might be relevant "It is seen that where the plaster of adjacent statues has been in intimate contact, not separated by even a millimeter of pit fill, presumed cycling of the calcium carbonate between carbonate and bicarbonate in response to moisture in the burial environment has resulted in the formation of a natural cement."
If it's not Neolithic cement, that could be one explanation, but why the coal dust? Alternatively, perhaps it's medieval or Roman cement?

What I took to be it, though it may not be, looked rather like some sort of hard licheny deposit. Similar stuff was also formed into a ring around the top of each of the uprights where they supported the capstone.
Second question: has anyone seen similar deposits round the uprights of other dolmens, and if so what is it?