Since this subject is now again under discussion by geologists, it may be of interest to recall that in my original paper, published in the American Journal of Geology, 1907, vol. xv, p. 23, and republished and revised in Harper's Monthly, 1907, vol. cxv, p. 120, and in the Scottish Geographical Magazine, 1907, p. 523, this separation was attributed to the great convulsion that occurred at the time of the birth of the Moon, from the side of the Earth. This explanation of the origin of our Moon is at the present time almost universally accepted by astronomers. We see the same phenomenon occurring in many close double stars.