I have from my boyhood been intimately acquainted with Snowdon and its neighbourhood, and for some years past I have been investigating the geology of the district in a systematic manner. In doing this I have paid particular attention to the lake basins, and shall in the following pages give an account of some of my investigations as far as they have gone. I say as far as they have gone, because I hope in the future to make by means of a level more accurate measurements than can be made with a pocket aneroid; and I also hope that next Summer the lakes and tarns will be sounded either by myself or by someone else; for at present we know nothing about the depth of Llyn dur Arddu, next to nothing above that of Glaslyn, and very little about Llydaw, though that little is of great importance.