"In our walk to Matlock, we passed along the side of the hill to Riber Top, where a singular assemblage of stones, supposed to have been originally a druidical altar; some antiquaries say, a cromlech, which appears more probable: they are called Hirst Stones, and are not unworthy of a visit; since those who feel no interest in these ancient relics will be amply repaid for the toil and trouble of ascending this eminence by the prospect it commands"
From 'Peak Scenery or The Derbyshire Tourist' 1824 by Ebenezer Rhodes.
Riber Hill is the place where Riber Castle was built - I remember going there as a kid, it had a very poorly looked after "zoo" type thing. It wasn't built until later than Rhiannon's source, in 1862, so may have been built over the top of the Druid's Altar (or even used the stone!).
Here's a bit about the castle (scroll down the page to "Riber"):
... These were destroyed a few years ago, for fencing, by a reckless individual, though abundance of stone lies scattered in all directions. A stone chair was preserved from the wreck by Mr. Gerge [?] Wall, which is now placed in his farm yard.
"On the eminence above Matlock Church, called Riber Hill, are the remains of what has been supposed a Druidical altar, but which has more resemblance to a cromlech; it is called the Hirst Stones, and consists of four rude masses of gritstone, one of which, apparently the smallest, is placed on the others, and is computed to weigh about two tons, on the upper side of which is a circular hole, made for the reception of a pillar in modern times..."
T. Bateman "Vesitiges of the Antiquities of Derbyshire.
From 'The Beauties of England and Wales' - J.Britton and E.W.Brayley, 1802:
On the eminence above the church, called Riber Hill, are the remains of what has been supposed a Druidical altar, but which has more resemblance to a cromlech; although it may only have been intended for a point to transmit signals.It is called the Hirst Stones, and consists of four rude masses of gritstone, on which apparently the smallest is placed on the others, and is computed to weigh about 2 tons. on the upper stone is a circular hole, wherein 50yrs ago stood a stone pillar