A complete lithology of the stones made by the late Professor J.W. Judd, in 1901, reveals the following rocks as comprising those used in the construction of Stonehenge.
1. Sarsens.—Coarse and fine-grained Sandstone similar to the Woolwich, Reading, or Bagshot beds. This stone is used for the Trilithons, Hele Stone, a recumbent stone known as the "Slaughtering Stone," and two small stones set north-west and south-east of the circle. It is of local origin.
2. Ophitic Diabase.—(Some porphyritic.)
3. Highly altered basic Tuffs, and agglomerates (calcareous chloritic schists).—Only one stump now remains.
4. Altered Rhyolites and Dacites.—Only fragments of this rock have been revealed during Mr. Gowland's excavations in 1901. At one time doubtless there was a whole upright of this material, but its striking appearance and fracture has probably led to its demolition by generations of souvenir hunters. Other fragments have been found in the barrows once within sight of Stonehenge, but now destroyed by cultivation.
5. Sandstones, Grits, and Quartzites.—The "Altar-Stone" belongs to this class. It is interesting to note that Professor Maskelyne has pointed out the similarity between the Altar Stone at Stonehenge, and the "Stone of Destiny" in the Coronation Chair at Westminster Abbey.
6. Grey Wackés.—Fragments only of these stones have been discovered among the chippings incidental to the dressing of the stones before erection.
Their absence at the present day is not a matter for surprise, as stones of this class weather badly, and when exposed to the action of frost and cold rapidly disintegrate.
7. Argillaceous Flagstones and Slates.—As in the case of the Grey Wackés, fragments only of these stones exist to tell the story of the uprights which have vanished under atmospheric changes.
8. Glanconitic Sandstone (possibly Upper Greensand?).—Traces of this rock have been discovered at Stonehenge by Mr. Cunnington. Professor Judd suggests the possibility of a boulder of this material having been found and used by the builders of Stonehenge.