During this autumn I had the opportunity of examining several different batches of celestine which had been dug out of the foundations of houses in this neighbourhood. On analysing one specimen which I took to be blue celestine, barium was found to be present in considerable quantities ; I then made a more exact qualitative analysis but failed to detect calcium or any other metal. The mineral then obviously consists of a double sulphate of barium and strontium, and a mineral of that kind has been found by Thomson, and was called by him baryto-celestine. Baryto-celestine, so far as I can ascertain, has not, however, been previously found in the British Isles.