Isolated mountains rising up like a huge lump, or a pen point, from a flat surface, are commonly found on the moon. These often stand several miles high and are particularly interesting as seen in the northern hemisphere of the moon, when the long shadow of such a solitary mountain stretches in inky-blackness against the smooth gray plain. Several of these may be seen in Mare Imbrium just above the crater Plato, the loftiest of which is Pico, rising about 8000 feet. Isolated mountains are very frequently found in the center of craters. This would seem a strange formation close at hand--a large crater, or a ringed plain, miles in extent, with a tall slender mountain standing in its center. The oddest and most common mountains, however, are the circular mountains which surround a plain or a large sunken cavity, like a mighty wall. Such mountains appear on almost every part of the moon's surface and in some places they appear in such profusion that their rings frequently touch and even overlap one another. The peaks of the circular mountains are often two or three miles in height and their shadows sometimes cover a large portion of the plain which they surround. These shadows are continually varying, for during the increase of the moon they are thrown in one direction and during the decrease, in the direction exactly opposite. During full moon they disappear altogether and the plains are filled with light, but lunar scenes are much more interesting when the sunlighted portions are accentuated by the long, queer shadows, for these shadows are black and clear-edged and stand out like silhouettes done in ebony, even at this distance of 240,000 miles. Because they have no light reflected into them from an atmosphere, these shadows are actually so densely black that if one stepped into such a shadow he would instantly be blotted from view.