4. BRIGHT STREAKS. These radiate prominently from many of the great craters of the Moon. They are streaks of narrow width but sometimes nearly a hundred miles in length. They are perhaps the most puzzling of all the Moon's features. They have been supposed by some observers to represent lava flows whose surface reflected light more brilliantly than other portions of the Moon. It is more generally believed, however, that the streaks do not represent any independent elevations, since they run over the highest mountains as well as through the deepest craters without variation.
GENERAL OBSERVATIONS.
The condition of the Moon's surface as a whole indicates that it has been a theater of extraordinary volcanic activity. In size and number its vulcanoids far exceed the volcanoes of the Earth. The largest terrestrial crater known is that of Kilauea in the Hawaiian Islands which is 2½ miles in diameter. Several craters of the Moon, however, exceed 50 miles in diameter and one measures 114¼ miles. While the absolute heights of the mountains of the Moon do not greatly exceed those of the Earth, proportionally they are much higher, since the Moon's diameter is only one-fourth that of the Earth. The vulcanoids of the Moon differ in other respects from the volcanoes of our globe. "On the Earth they are usually openings on the summits or sides of mountains—on the Moon, depressions below the adjacent surface even when it is a plain or valley; on the Earth the mass of the cone usually far exceeds the capacity of the crater — on the Moon they are much nearer equality; on the Earth they are commonly the sources of long lava streams—on the Moon, traces of such outpourings are rare." (Webb.)
DESCRIPTION OF INDIVIDUAL FEATURES.
(Abridged from Nasmyth and Carpenter.)
The numbers refer to those on the accompanying chart.
Copernicus. 147. This may deservedly be considered one of the grandest and most instructive of lunar craters. Though its diameter (46 miles) is exceeded by that of other craters, its situation near the center of the lunar disc renders it so conspicuous as to make it a favorite object for observation. Its vast rampart rises to upwards of 12,000 feet above the level of the plateau, nearly in the center of which stands a magnificent group of cones attaining the height of upwards of 2400 feet. The rampart is divided by concentric segmented terraced ridges, which present every appearance of being enormous landslips, resulting from the crushing of their overloaded summits which have slid down in vast segments and scattered their debris on the plateau. Corresponding vacancies in the rampart may be observed from whence these prodigious masses have broken away. The same may be noticed, to a somewhat modified degree, around the exterior of the rampart. For upwards of 70 miles around Copernicus myriads of comparatively minute but perfectly formed craters can be seen. The district on the southeast side is especially rich in them. Many somewhat radial ridges or spurs may be observed leading away from the exterior banks of the great rampart. They appear to be due to the freer egress which the extruded matter found near the focus of disruption.
Triesnecker. 150. A fine example of a normal lunar volcanic crater having all the usual characteristic features in great perfection. Its diameter is about 20 miles and it possesses a good example of the central cone and also of interior terracing. The most notable feature, however, is the remarkable display of cracks or chasms which may be seen to the west side of it. Several of these cracks obviously diverge from near the west external bank of the great crater and they sub-divide or branch out as they extend from the apparent point of divergence, while they are crossed or intersected by others. These cracks or chasms are nearly one mile wide at their widest part and after extending for fully 100 miles taper away till they become invisible.
Theophilus. 97. Cyrillus. 96. Catharina. 95. These three magnificent craters form a conspicuous group. Their diameters and depths are as follows: Theophilus, diameter, 64 miles; depth of interior plateau from summit of crater wall, 16,000 feet; central cone, 5200 feet high; Cyrillus, diameter, 60 miles; depth of interior plateau from summit of crater wall, 15,000 feet; central cone, 5800 feet high; Catharina, diameter, 65 miles; depth of interior plateau from summit of crater wall, 13,000 feet; center of plateau occupied by a confused group of minor craters and debris. Each of these craters is full of interesting details presenting in every variety the characteristic features of the lunar volcanoes and giving unmistakable evidence of the tremendous energy which at some remote period piled up such gigantic formations. The intrusion of Theophilus within Cyrillus shows that it is of more recent formation than the latter. The flanks of Theophilus, especially on the west side, are studded with apparently minute craters. These would be considered of great size but for the enormous crater so near.
Ptolemy. 111. Alphons. 110. Arzachael. 84. The portion of the moon's surface which includes these features, being near the center of the lunar disc, is exceptionally well placed for observation. Within this area may be seen every variety of volcanic craters and a number of other interesting forms. Ptolemy belongs to the class of walled plains, its ramparts enclosing a plain 86 miles in diameter. Alphons and Arzachael are respectively 60 and 55 miles in diameter. They have all the distinctive features of lunar craters, viz:—central cones, lofty, ragged ramparts, manifestations of landslip formations in the great segmental terraces within their ramparts and minor craters interpolated within their plateaus. A notable object near Alphons is an enormous straight cliff traversing the diameter of a low, ridged, circular formation. This great cliff is 60 miles long and from 1000 to 2000 feet high. It is a well known object to lunar observers and has been termed "The Railway" on account of its straightness. The existence of this remarkable cliff appears to be due either to an upheaval or a down-sinking of a portion of the surface of the circular area across whose diameter it extends.