[Footnote 63: The literature of Anglesey geology is now somewhat voluminous, but I may refer to the following as the chief authorities. The island is mapped in Sheet 78 of the Geological Survey of England and Wales, and its structure is illustrated in Horizontal Sections, Sheet 40. A full account of its various formations and of their relations to each other is given in vol. iii. of the _Memoirs of the Geological Survey_, "The Geology of North Wales," by Sir A. C. Ramsay, 2nd edit. 1881. The subject has been discussed by Professor Hughes, _Quart. Journ. Geol. Soc._ vols. xxxiv. (1878) p. 137, xxxv. (1879) p. 682, xxxvi. (1880) p. 237, xxxviii. (1882) p. 16; _Brit. Assoc. Rep._ (1881) pp. 643, 644; _Proc. Camb. Phil. Soc._ vol. iii. pp. 67, 89, 341; by Professor Bonney, _Quart. Journ. Geol. Soc._ vol. xxxv. (1879) pp. 300, 321; _Geol. Mag._ (1880) p. 125; by Dr. H. Hicks, _Quart. Journ. Geol. Soc._ vols. xxxiv. (1878) p. 147, xxxv. (1879) p. 295; _Geol. Mag._ (1879) pp. 433, 528 (1893) p. 548; by Dr. C. Callaway, _Quart. Journ. Geol. Soc._ vols. xxxvii. (1881) p. 210, xl. (1884) p. 567; and by the Rev. J. F. Blake, _Quart. Journ. Geol. Soc._ vol. xliv. (1888) p. 463. Further references to the work of these observers in Anglesey are given in Chapter xiii. p. 220 _et seq._ The Pre-Cambrian areas of Anglesey are shown in Map II.] There are two groups of rocks in Anglesey to which a pre-Cambrian age may with probability be assigned. In the heart of the island lies a core of gneiss which, if petrographical characters may be taken as a guide, must certainly be looked upon as Archæan. In visiting that district with my colleague Mr. Teall I was much astonished to find there so striking a counterpart to portions of the Lewisian gneiss of the north-west of Sutherland and Ross. The very external features of the ground recall the peculiar hummocky surface which so persistently characterizes the areas of this rock throughout the north-west of Scotland. If the geologist could be suddenly transported from the rounded rocky knolls of Sutherland, Ross-shire or the Hebrides to those in the middle of Anglesey, south of Llanerchymedd, he would hardly be aware of the change, save in the greater verdure of the hollows, which has resulted from a more advanced state of decomposition of the rocks at the surface, as well as from a better climate and agriculture.