In a study of the surface features of our satellite one is immediately struck by the remarkable sharpness and freshness which most of those features exhibit. Further, one is impressed by the fact that evidences of compressive action are in the minority, whereas the moon's crust shows clearly innumerable clefts, rifts, and fractures pointing to an extensional tendency of its outer envelope. This is hardly in accordance with our ideas of the appearance of a sphere which has lost all its heat; everywhere it should show signs of intense contraction, and arcuate mountain ranges should be more frequent than upon the face of the earth.