In it he argues that Stonehenge like Avebury were places that were reused by the romans for worship, a cult might have existed.
One religious belief system supplanting, suppressing or merging with another is likely to say to the established religion, "Ah, where we come from we call that god/goddess by this name but it's obvious he/she are one and the same." The natives nod and they all go off for a jar or two down by the waterfront. Is there a common root/tradition I wonder in the names Salisbury, Silbury and Aquae Sulis... 