These are rather level plains of an oval or circular outline, enclosed by a wall of mountains.
The finest example is, perhaps, the dark-gray Plato, situated in 50 deg.

of north latitude, near an immense mountain uplift named the Lunar Alps, and on the northern shore of the _Mare Imbrium_, or "Sea of Showers." Plato appears as an oval plain, very smooth and level, about 60 miles in length, and completely surrounded by mountains, quite precipitous on the inner side, and rising in their highest peaks to an elevation of 6,000 to 7,000 feet.

Enclosed plains, bearing more or less resemblance to Plato--sometimes smooth within, and sometimes broken with small peaks and craters or hilly ridges--are to be found scattered over almost all parts of the moon.

If our satellite was ever an inhabited world like the earth, while its surface was in its present condition, these valleys must have presented an extraordinary spectacle.

It has been thought that they may once have been filled with water, forming lakes that recall the curious Crater Lake of Oregon.
[Illustration: THE MOON AT FIRST AND LAST QUARTER (WESTERN AND EASTERN HEMISPHERES).