I. Introduction. Twenty-two years ago, a paper by me on ‘Some New Points in the pre-Cambrian Geology of Anglesey’ appeared in the Geological Magazine.‘ One section of that paper discussed the ’ Continuity of the Granitoid Series with an underlying Gneiss,‘ and the chief evidence for that continuity was furnished by the Craig-yr-Allor anticline. An ellipsoidal dome of hornblendic and chloritic gneiss in Central Anglesey was described as surrounded by ’granitoidite,‘ below which it appeared to dip in a quaquaversal manner. As both the gneiss and the granitoid rock were then generally regarded as of sedimentary origin, the inference that the gneiss represented a formation older than the ‘granitoidite’ appeared to be justified. We now know, however, that these rocks are not metamorphosed sediments, but plutonic masses; therefore they can never have been in stratigraphieal sequence. A review of the ground in the light of the new theory of origin has now been undertaken, and the following paper is the result. I include within its scope the acidic rocks, both massive and schistose, which are associated with the gneisses and granite, so as to present a general outline of the structure of the plutonic complex of Central Anglesey. I do not propose to enter into minute petrographical or geognostic details : the former being unnecessary for my purpose, while the latter would be superseded by the expected mapping of Mr. :Edward Greenly. Some corrections were made in the old interpretations of the district by the Rev. J.F. Blake “ in