Of these 7,300,000 square miles, about 2,900,000 square miles are occupied by the gray, or dusky, expanses, called in lunar geography, or selenography, _maria_--i.e., "seas." Whatever they may once have been, they are not now seas, but dry plains, bordered in many places by precipitous cliffs and mountains, varied in level by low ridges and regions of depression, intersected occasionally by immense cracks, having the width and depth of our mightiest river canons, and sprinkled with bright points and crater pits.

The remaining 4,400,000 square miles are mainly occupied by mountains of the most extraordinary character.

Owing partly to roughness of the surface and partly to more brilliant reflective power, the mountainous regions of the moon appear bright in comparison with the dull-colored plains.
Some of the lunar mountains lie in long, massive chains, with towering peaks, profound gorges, narrow valleys, vast amphitheaters, and beetling precipices.

Looking at them with a powerful telescope, the observer might well fancy himself to be gazing down from an immense height into the heart of the untraveled Himalayas.

But these, imposing though they are, do not constitute the most wonderful feature of the mountain scenery of the moon.
Appearing sometimes on the shores of the "seas," sometimes in the midst of broad plains, sometimes along the course of mountain chains, and sometimes in magnificent rows, following for hundreds of miles the meridians of the lunar globe, are tremendous, mountain-walled, circular chasms, called craters.