In the following notes we propose to collect descriptions of some of the igneous rocks on N. Wales founded on the collections made by the late Prof. Sedgwick many years ago. It is hoped that it may be of interest to readers of this Magazine, as many students turning their attention to rocks find a want of information in text-books concerning precise localities in Britain for the occurrence of many interesting types. Following Sedgwick's steps may help them partly to what they require, while the microscopical analyses appended will doubtless prove an aid to beginners, besides being a definite record, as far as it goes, of some types of Welsh rocks.