After highlighting the contrast between the dark plains and the bright mountainous country, let us look at the mountain masses themselves, and again only at their general structure.

The first characteristic feature which stands out in a telescopic view of the Moon, even to the most untrained observer, is the circular shape.

It repeats itself in many thousands of examples, of which the largest mountains may be amongst the largest formations on our neighboring world, but also lesser formations, whose extreme smallness escapes our certain detection.

It is unmistakable on some well-delineated grey areas (Mare) there is the unmistakable tendency towards the circular enclosure, the resemblance to the Ring mountains.

Where several plains touch each other, their often mighty boundary walls in large bays show parts of a circular arc, which, interrupted here, appears to be continued in another mountain range.

Let us agree with this view, which is supported by the nature of the boundaries of Mare Crisium, Mare Serenitatis, Nectaris, Humorum, the large mare-like ramparts Kästner and Mare Humboldtianum - after all, this marvellous inclination to the circular shape, which is distinguished by the greatest differences in the mare, is not to be overlooked, and it is very probable that they should not be regarded as purely accidental.

It gives us an idea that in ancient times the first and most formidable catastrophes on the Moon had shaped these very forms, many thousands of others of which have followed on a smaller scale and in innumerable passages.

It will be advantageous, for the sake of clarity, first to describe the actual crater-mountains, and only later in particular the mass-mountains (Massif) and the isolated heights of the Moon.