How about this -
Jones saw Stonehenge as a Roman temple (to Coelus/Uranus) but not all Roman altars are upright, some are horizontal on plinths. Did he give an opinion on if this was once upright, or is that just something he has been assumed to have thought???? Suppose he knew or believed it was once mounted horizontally in a low trilithon fashion? That would be a very strong reason for him to think it was an altar.
And I bet he knew his Roman Altars. I think he visited Italy, and he was deep into Renaissance stage design and worked on Ben Jonson's court masques. There were a lot of productions on classical themes involving scenes in temples and whatnot. Maybe some details survive - and exactly what was his conception of a Roman altar is still available to be found?
