ON MAPPING THE SURFACE OF THE MOON. 79 



Assoc. 1865, p. 304) appears to be a depressed surface S. of the cleft of 

 Ariadaus, the N. side being at a higher level. In like manner a portion of 

 the surface N.E. of the line of chffs ivom. Ptohmceus to Bitter andL. Sabine va^j 

 have subsided and produced the depressed region known as Hipparclius." 



" While areas of depression, if not of subsidence, can be traced on the 

 surface of the moon, and also the presence of a material which has invaded 

 such regions and in many instances nearly buried preexisting craters and 

 other objects, it is not so easy to ascertain whence this material came ; still 

 closer scrutiny is indispensable to throw further light upon it." 



" In numerous portions of the moon's surface, as on that of the earth, we 

 behold the results of the operation of two opposing forces, — one by which the 

 features are moulded and, as it were, built up, imparting to the objects so 

 produced an aspect of freshness that it is impossible to question their com- 

 parative recent production ; the other by which objects once possessing all 

 the characteristics of a recent formation have yielded, it may have been 

 gradually, to surrounding influences, whatever they may have been, so that 

 at the present time they exhibit the semblance of vast ruins, which in some 

 localities are unrelieved by even the slightest indication of the operation of a 

 force of an opposite character." 



" Webb, in his very masterly paper on the Moon, in ' Eraser's Magazine ' for 

 September 1868, speaks of the possibility that the colossal lunar formations 

 may have been the result of forces acting in a more gradual manner and 

 with less temporary vehemence than may seem to comport with the term 

 explosion. It may be that astronomers may have paid much more attention 

 to those lunar features which are clearly the results of explosive action than 

 to those which manifest the presence of a degrading agency. It has been 

 considered that many of the larger forms have been produced by rapid, 

 violent, and tumultuary processes ; and, however true this view may be, it 

 is certainly inadeqiiate to account for the present appearances of still larger 

 tracts in which no explosive outburst of an epoch which may in any sense 

 be called recent occurs. Nearly filled as well as broken rings, interrupted 

 mountain-chains, and comparatively smooth tracts without any weU-defined 

 boundaries are characteristic of such regions ; and it may be asked in what 

 manner and by what agency have they attained their present condition? 

 Has the ' erosion ' of Chacornac destroyed the missing portions of the broken 

 rings ? and has this 'erosion' acted suddenly or gradually'l Has the 'diluvial,' 

 restricted by Webb to the expression of comparative fluidity, independent of 

 the nature of the material, invaded and nearly fiUed previously deep craters, 

 so as to furnish a connected series of well-known forms, from the smooth, 

 floored, waUed plain to the just perceptible ring above the surface? Has 

 this same ' dUuvial ' buried the lower i^ortions and the lateral spurs of 

 continuous mountain-chains, so that now the higher portions alone remain 

 as short and detached ranges in the original line ? One cannot help con- 

 trasting the continental region, to use a terrestrial analogy, in which this 

 area IV A*" occurs, with the magnificent chains of the Apennines and Hcemus, 

 and the lower and smooth levels of the 2Iare Imhrium and Mare Serenitatis, 

 as exhibiting in a very marked degree the results of the forces already men- 

 tioned. In the latter we see the effects of comparative recent action in the 

 production of vast mountain-chains and the neighbouring extensive level 

 plains. In the former these grand features are wanting; the surface, 

 although far from being smooth as that of the Maria, is roughened only with 

 the remains of former mountains, rings, and craters ; the degrading agency, 

 whatever it may have been, appears to have operated almost unchecked in 



