[Introductory Note.—After the publication of my paper on the Sarsens, or Sarsen Stones, in the Wiltshire Archæological and Natural History Society's Magazine, vol. xxiii (1886), pp. 122–154, many friendly communications gave me further information on the subject, and additional references to published facts and opinions. From this correspondence, and my own notes made in the country, I propose to utilize much that seems to be of interest. The most important of these additions to our knowledge of the Sarsens is the following memoir on their constitution and structure by my friend Prof. Dr. J. W. Judd, C.B., F.R.S., etc., of the Royal College of Science, who most obligingly examined with care the microscopical structure of many specimens from authenticated localities. With his kind permission this valuable communication (dated March 9th, 1888) is here printed.—T. RupertJones.]