Stukeley's prose is actually so beautiful as to be poetic. Rearranged it might look like this -

About three foot below the surface,
a layer of flints...
about a foot thick,
rested on a layer of soft mould another foot:
in which was inclos’ed an urn full of bones...
The bones had been burnt,
and crouded all together in a little heap,
not so much as a hat would contain...

We made a cross-section ten foot each way,
three foot broad over its center...
At length we found a squarish hole cut into the solid chalk,
in the center of the tumulus.

It was three foot and a half,
i.e., two cubits long,
and near two foot broad,
i.e. one cubit: pointing to Stonehenge directly.
It was a cubit and a half deep from the surface...
Regarding one of the small ones, 20 cubits in diameter...

A child’s body (as it seems) had been burnt here,
and cover’d up in that hole:
but thro’ the length of time consum’d.
From three foot deep,
we found much wood ashes soft and black as ink.