Another of the prominent features which we notice as highly curious, and in some phases of the moon—at about the time of full—the most remarkable of all, are certain bright lines that appear to radiate from some of the more conspicuous craters, and extend for hundreds of miles around. No selenological formations have so sorely puzzled observers as these peculiar streaks, and a great deal of fanciful theorizing has been bestowed upon them. As we are now only glancing at the moon, we do not enter upon explanations concerning them or any other class of details; all such will receive due consideration in their proper order in succeeding chapters.
We thus see that the classes of features observable upon the moon are not great in number: they may be summed up as _craters_ and their central cones, _mountain chains_, with occasional isolated peaks, _smooth plains_, with more or less of irregularity of surface, and _bright radiating streaks_. But when we come to study with higher powers the individual examples of each class we meet with considerable diversity. This is especially the case with the craters, which appear under very numerous variations of the one order of structure, viz., the ring-form. A higher telescopic power shows us that not only do these craters exist of all magnitudes within a limit of largeness, but seemingly with no limit of smallness, but that in their structure and arrangement they present a great variety of points of difference. Some are seen to be considerably elevated above the surrounding surface, others are basins hollowed out of that surface and with low surrounding ramparts; some are merely like walled plains or amphitheatres with flat plateaux, while the majority have their lowest point of hollowness considerably below the general level of the surrounding surface; some are isolated upon the plains, others are aggregated into a thick crowd, and overlapping and intruding upon each other; some have elevated peaks or cones in their centres, and some are without these central cones, while the plateaux of others again contain several minute craters instead; some have their ramparts whole and perfect, others have them breached or malformed, and many have them divided into terraces, especially on their inner sides.