in waves that produce a vibratory effect upon the sand surface. The noise comes to one then in strikingly rhythmical fashion, I was also aware of a distinct, though delicate and peculiar, vibration which seemed to come from the sand and affected my whole body when I stepped upon a fresh place and agitated the sand for the first time. I do not know whether this came from the crunching of dry sand particles under my feet or whether it is a matter of electrical tension, In any case, I had no instru- ments to measure it and only record the impression. The volume of the sound clearly depends on the state of the sand, whether it is piled ready to slide down at a touch on the steeper slopes of the dune or whether it lies on slightly flatter grades. At the time of an earthquake great masses slide down over each other as the trembling of the earth dislodges sand that the wind has piled up for days or months beforehand, This causes the vibration to be much louder and of longer amplitude, and it may then be heard a mile or so away. Naturally this has given rise to superstitions of one sort and another, and popular explanations given in the valley are that the noises emanate from caves in the mountain or from some concealed volcanic crater from which an eruption may some day arise.
