What does Stony Sleep mean?

It is a reference to William Butler Yeats' poem 'The Second Coming'. Below is shown a copy of the poem with some notes as to the interpretation which I pinched from somewhere (think it was some lecture on trauma therapy which had used it for an introduction).

From what I've been told and read, it is essentially a prediction that after 'Twenty Centuries' (or the crucifixion), the 'Second Coming' (the coming of the antichrist) will be upon us.

I am not particularly knowledgable about the poem, but I thought many wouldn't know where the name Stony Sleep came from and might be interested

If anyone has problems with me posting this particular image please tell me and I will find a workaround for the page. It just seemed like a helpful interpretation for the poem in one