I. I ntroductory R emarks . T he northern part of Anglcsey contains a group of rocks regarding the age of which geologists hold divided opinions. As the well-defined district in which they lie occupies the north of the island,the strata taken as a whole are known as the ‘Northern District Rocks.’ From their frequently twisted and contorted aspect, Prof. Hughes calls them the ‘Gnarled Series.’ These beds have a general dip to the north or north-east, and cosist mainly of green chloritie schists and grits, and greenish-grey and purple phyllites, slates, and shales. They are everywhere bounded on the south by black slates and shales of acknowledged Ordovician age. This boundary-line forms a curve running from coast to coast from near Carmel Head on the west through Llanfflewin to Perth y Corwgl on the east ; it is shown on the 1-inch Survey map, and is considered by several geologists (among others, Ramsay, Hicks, Callaway, and J. F. Blake) as a line of dislocation—an ordinary fault according to Ramsay, 1 or according to later writers as a thrust whereby the Northern District rocks have been pushed over the Ordovician beds. Ramsay 2 believed this northern area to be mainly altered Cambrian strata, with some Lower Silurian (Ordovician) beds in the northernmost parts near Cemaes, 3 though he could not here separate the reeks of the two systems. 4 Dr. Iticks 5 considered the beds north of the curved fault to be pre-Cambrian (Pebidian) ; Dr. Callaway 6 also classified the beds as pre-Cambrian