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 at the base, we climbed a spur of the mountain to the summit and obtained a general view of the valley (Fig. 46). he upper- most sand dine extends to within 1oo feet of the summit. There is about 300 feet difference of clevation between the base and the top of the dune which lies in a natural hollow between two rocky spurs. It is perhaps 500 feet wide. The sand has been blown from the dry river bed and valley Hoor. It is net locally derived. The mountain itself is made of a dark volcanic rock, The sand is a light-colored quartz sand like that which is strewn so abundantly on the lower slopes of the valley and its floor.

We walked down upon the dune surface without hearing any noise whatever, the day being perfectly quiet. 1 had begun to doubt the reputation of the mountain, when | thought of starting a larger mass of sand in motion by rocking back and forth on my heels. The sand is piled up at an inclination of 30°, or about the “angle of repose,” and it is therefore not difficult to send considerable masses of it rolling down the steep incline. Started at the top a quantity of sand will roll at least halfway and sometimes the whole distance down the dune.

As soon as I started a mass of sand moving down the slope there came up to me a distinct vibrating hum as nearly as I can describe it. It was about as loud as that which one would make in humming an air to oneself. When larger masses of sand were detached and sent rolling down the incline the hum increased in volume, and the crests of the sound waves under these circumstances seemed to fall about a second apart. I then tried pitching stones upon the surface of the dune, and when these fell in the steepest portion so that sand started rolling the same effect was produced. It is said that the sand will not make a noise when it is damp from the fog of winter. Though all of it was relatively dry, I was able to find some of it that contained a distinct amount of moisture. On experiment-