The extraordinary wealth of the formations, or, to avoid here a geological expression [59] of the forms on the Moon, makes a coherent and yet reasonably clear description very difficult.

What is always advisable, is to maintain the Classification of the Moon mountains, introduced by SCHRÖTER, LOHRHANN, and MÄDLER, so one does not fail to feel the variability in the systematic sequencing of manifestations, which are more or less related to one another by infinitely different intermediate and transitional elements.

The composition, as well as horizontal and vertical dimensions, are what will occupy us mainly in the following sections; the descriptive selenography has to do only with this object of observation; one can engage in further assumptions, but one also leaves the solid ground of a science whose first foundations are to be found in numerical determinations.

Thus, while for the time being we are only concerned with the sure part of the observation of the Moon, it is necessary first to give the means by which one obtains knowledge of the height of the mountains.

The height ratios on the Moon become of greater importance the more one approaches the objective, which is to provide an explanation of the origin of the lunar mountains and the special comparison of these with the mountains of the Earth.

The method of height measurements and the certainty of their results, proven by special examples, must precede the description of the mountains.