A NEW CRATER IN THE MOON. 105 



continent ; in fact, it may be doubted whether as yet the 

 materials for such a description exist. 



The mountain-ranges of the moon do not differ to any 

 marked degree from those of our own earth. They are few 

 in number ; in fact, mountain-ranges form a less important 

 feature of lunar than of terrestrial geography. On the other 

 hand, the lunar ring-mountains and craters far exceed those 

 of our earth in size and importance. The large craters may, 

 in fact, be regarded as characteristic features of lunar scenery. 

 There are several craters exceeding 100 miles in diameter. 

 It is strange to consider that though the ringed wall sur- 

 rounding some of these larger craters exceeds 10,000 feet in 

 height, no trace of the highest peaks of such a wall would 

 be visible from the middle of the enclosed plain. Con- 

 versely, an observer standing on one of the highest peaks 

 beside one of these great craters, would not see half the floor 

 of the crater, while more than half the horizon line around 

 him would belong to the enclosed plain, and would appear 

 as level as the horizon seen from a height overlooking a great 

 prairie. These ringed plains and larger craters seem to be- 

 long to the third great era of the moon's history. The bright 

 high regions and dark low levels called seas must have been 

 formed while the greater part of the crust was intensely 

 hot. The contraction of the cooling crust on the nucleus, 

 which cooled far less slowly, led to the formation of the great 

 ray systems. But though such systems extend from great 

 craters, these craters themselves probably attained their pre- 

 sent form far later. The crust having in great part cooled, 

 the nucleus began in turn to shrink more quickly than the 

 crust, having more heat to part with. Thus the crust, closing 

 in upon the shrinking nucleus, formed the corrugations and 

 wrinkles which can be seen under telescopic scrutiny in 

 nearly every part of the visible lunar surface. The process 

 was accompanied necessarily by the development of great 

 heat the thermal equivalent of the mechanical process of 

 contraction. Mallet has shown that the process of con- 

 traction at present occurring in the earth's crust gives rise to 



