PORTION OP THE COSMOS. THE PLANETS. 361 



them, the not very definitely bounded Oceanus Procellamm has 

 the greatest extent (90000 German geographical square miles). 

 In connection with the Mare Imbrium (16000 German 

 square miles), the Mare Nubium, and in some degree with the 

 Mare Humorum, and inclosing insular highlands (the Ri- 

 phasan Mountains, Kepler, Copernicus, and the Carpathians), 

 the eastern darker portion of the Moon's surface forms the 

 most decided contrast with the more brightly beaming south- 

 western region, in which mountain is crowded against moun- 

 tain ( 583 ). In the north-western region there are two more 

 detached and isolated basins the Mare Crisium (3000 

 German geographical square miles), and the Mare Tranquil- 

 litatis (5800 such miles). 



The colour of these so-called seas is not always grey. 

 The Mare Crisium has a grey tint mixed with dark green, 

 and the Mare Serenitatis and Mare Humorum are likewise 

 green. Near the Hercynian Mountains, the isolated part 

 inclosed within the circumvallation called Lichtenberg, has, 

 on the other hand, a pale-red tint ; and so, also, has Palus 

 Somnii. Annular depressions without central mountains 

 have most often a dark steel-grey tint verging towards 

 bluish. The causes of these different tints of colour in 

 the rocky or other less coherent substances forming the 

 surface, are exceedingly enigmatical. 



A great wall-surrounded plain called Plato (by Hevelius, 

 Lacus niger major), on the north of the " Alps/' and still 

 more, Grimaldi in the equatorial region, and Endymion, near 

 the north-western margin, are the three darkest places in 

 the whole lunar disk ; and the brightest of all is Aristar- 

 chus, of which, when it is on the portion of the Moon not 



