Having looked closer I can now answer my own question;
1 the severely rotten joists are old wet rot hence no cubic cracks
2 the rest is later (recent ) dry rot
3 I've found the telltale cubic cracks in a small area under the joist
at about the center i.e. oldest, part of the dry rot, as though it
started in one place and has spread out from there.

cheers

Jacob

Chapel conversion:
Have an area of dry rot in my basement floor - at least it has all the
symptoms - mycelium like cotton wool or fine cobwebs, orange dust
spores, brown wood, fruiting body (just one small one) but no
cuboidal cracking.
At its worst there are 3"x5" joists gone dark brown with distorted
surface, which are as light as balsa wood and you can break them in
half over your knee.
Does absence of cuboidal cracking mean a different sort of rot - less
persistent I hope?

cheers

Jacob