In the Woodwardian Museum is a collection of nearly a thousand rock-specimens made by Professor Henslow to illustrate his “ Geological Description of Anglesea ” (1821, Trans. Camb. Phil. Soc. vol. i. pp. 359–452, plates xv.–xxi.). Specimens of the principal dykes were submitted to the examination of Professor Cordier, and his remarks on them are quoted in Henslow's paper. As Cordier's determinations date from a time when thin slices of rocks were unknown, and but little attention has since been given to the dykes of Anglesey, it is believed that brief notices of some of the typical rocks from localities easity identified may have an interest for British geologists. As regards the mode of occurrence of the dykes and their effects upon the adjacent strata, little can be added to the accurate descriptions of Henslow, written at a time when the igneous origin of dykes was a proposition to be proved.