Last summer I had the pleasure of passing a fortnight in the Channel Islands under the guidance of my friend the Rev. E. Hill, who has done so much to elucidate their geology. During our short visit to Sark we spent some hours in the beautiful little cove called Port du Moulin, examining the interesting sections of the hornblende-schist and underlying gneissic rocks. I was wandering on the beach looking at the wave-worn boulders which afford most interesting studies of the structure of this crystalline series, when my eye was attracted by one which differed much from the rest, and resembled a dark-coloured serpentine of a slightly exceptional character. With some difficulty, owing to its form, I detached a tolerable specimen, and on examining the fresher surface, felt convinced that I had found a rock composed chiefly of an altered olivine and a silvery talc-like mineral.