issued from its side. These various circumstances, taken together, lead to the inference that the sand may have originated in the decomposition of the rock beneath. It is further noticeable, that the _Jabel Nakous_ is composed of a white friable sandstone, resembling that of the white friable bed of the Bay of Laig, and that it belongs to nearly the same geological era. I owe to the kindness of Dr. Wilson of Bombay, two specimens which he picked up in Arabia Petraea, of spines of Cidarites of the mace-formed type so common in the Chalk and Oolite, but so rare in the older formations. Dr. Wilson informs me that they are of frequent occurrence in the desert of Arabia Petraea, where they are termed by the Arabs petrified olives; that nummulites are also abundant in the district; and that the various secondary rocks he examined in his route through it seem to belong to the Cretaceous group. It appears not improbable, therefore, that all the sonorous sand in the world yet discovered is formed, like that of Eigg, of disintegrated sandstone; and at least two-thirds of it of the disintegrated sandstone of secondary formations, newer than the Lias. But how it should be at all sonorous, whatever its age or origin, seems yet to be discovered. There are few substances that appear worse suited than sand to communicate to the atmosphere those vibratory undulations that are the producing causes of sound: the grains, even when sonorous individually, seem, from their inevitable contact with each other, to exist under the influence of that simple law in acoustics which arrests the tones of the ringing glass or struck bell, immediately as they are but touched by some foreign body, such as the hand or finger. The one grain, ever in contact with several other grains, is a glass or bell on which the hand always rests. And the difficulty has been felt and acknowledged. Sir John Herschel, in referring to the phenomenon of the _Jabel Nakous_, in his "Treatise on Sound," in the "Encyclopaedia Metropolitana," describes it as to him "utterly inexplicable;" and Sir David Brewster, whom I had the pleasure of meeting in December last, assured me it was not less a puzzle to him than to Sir John. An eastern traveller, who attributes its production to "a reduplication of impulse setting air in vibration in a focus of echo," means, I suppose, saying nearly the same thing as the two philosophers, and merely conveys his meaning in a less simple style.