PLATE XU. 



HYGINUS AND THE NEIGHBORING FIELD. 



This plate is intended to show the general character of the area in which lie the Hyginus 

 clefts. It should be noted that parallel and near to those of Hyginus there is another which also 

 intersects a vulcanoid. It is less perfect but evidently of the same nature. A yet more indistinct 

 object of the same nature lies near the west wall of the large crater north of Hyginus. 



The group of mountains lying near Hyginus shows the elongate character which those ridges 

 often assume. In other parts of the field they are distinctly conical. Near the clefts is a good 

 example of crater valleys. Others less distinct lie near the southern border. A large vulcanoid 



near the margin of the plate has evidently 

 had a part of its rim broken down, probably 

 by the lava of the neighboring mare. 



The difference between the features 

 shown in this plate and the drawing figured 

 herewith will serve to show the reader how 

 diverse are appearances of the moon's surface 

 under different conditions of observation. 



This drawing may be compared with the 

 photographs of the same object (Pis. XII, 

 XX11) to show the relative minuteness of de- 

 tail grasped by a photograph and by the eye. 

 It shows the vulcanoid Hyginus with the re- 

 markable clefts which proceed from it as 

 exhibited in a drawing. The crater is in 

 no wise exceptional, except for the fissures 

 which break through its encircling wall and 

 extend for a great distance on either side. 

 These are among the most instructive of this 

 group of lunar features. 



It should be noted that the general con- 

 tour of the walls on either side of the clefts 

 indicates that a number of small craters were 

 first formed and then divided by the formation of the vent and the separation of its walls. That 

 such was the case is well shown by the fact that the cleft on the right has a part of the ring of at 

 least four of these small vulcanoids on one side of its wall and a part on the other. There is 

 a faint trace of the same feature in the rift on the left of Hyginus. A like separation has taken 

 place in the walls of the principal crater. The fact that the floor of this crater is apparently not 

 divided probably indicates that it was molten at the time when the rupture occurred, or that it 

 afterwards was so. ' The level surface of the bottom of the clefts can best be explained by sup- 

 posing that they, too, are floored by lava which entered them at some time after they were formed. 

 It is probable that this lava came from the depths, for the reason that, as elsewhere noted, there 

 is reason to believe that the lunar lavas were not sufficiently fluid to flow readily. (See p. 12.) 

 The facts appear to indicate that this crevice was formed before the interior of the moon had 

 ceased to be fluid. 



1 Elger states that he has seen, though faintly, traces of the cleft crossing the floor of the crater. If this observation 

 was well made, then we have to suppose that the lava did not quite fill this part of the rift, which does not appear on 

 this drawing, though it exhibits features that Elger had evidently not observed. Such developments are very common in 

 sketches of lunar structures. 



HYGINUS FROM A DRAWING WITH THE IS-INCH EQUATORIAL 



AT THE ALLEGHENY OBSERVATORY. F. W. VERY, DEL., 



1890. THREE-EIGHTHS SIZE OF ORIGINAL DRAWING. 



104 



