﻿relative to Lunar Activity. 187 



nearly opposite the one to the other ; and the inflexions of the 

 curves, which in the early portions resemble those of Nos. 19 

 and 13, are nearly similar. This similarity is not, as in the case 

 of Nos. 16 and 5, explicable upon the presence of a fault ; ne- 

 vertheless the four spots agree in being situated near the border. 

 Now it has been ascertained by careful observation under oblique 

 illumination, that the floor of Plato dips towards the border as if 

 a fissure existed just within it. The similarity of the two sets 

 of curves, the spots being near the border, is remarkable, and 

 goes far to connect in some way the phenomena of visibility with 

 the localities of the spots. 



While upon this part of the subject, it may be well to solicit 

 attention to the remarks of Scrope and Hopkins relative to the 

 formation on the earth of tracts similar to Plato on the moon. 

 If with Scrope we suppose the expansion of subterranean beds 

 of crystalline or other rock to have taken place at a great depth, 

 under a a } fig. 2, elevating the overlying strata, and, with Hop- 

 kins, that when the surfaces tbus elevated attained a condition 



Fie:. 2. 



at which the tension and cohesion just balanced each other, the 

 slightest increase of tension would rupture the surface and pro- 

 duce two systems of fissures, which might be considerably aug- 

 mented by earthquake-waves accompanied by the sudden subsi- 

 dence of the tracts between the two principal lines of fissures, — 

 we have the conditions for the production of such a region as 

 Plato, the annular fissure opening outwardly being indicated by 

 the dip of the floor to the border. Within and below the area, 

 however, another fissure must occur, as at s, opening in the re- 

 verse direction, i. e. towards the lava which may exist in the 

 interior, and which would ascend in the fissure and perhaps 

 force its way through some minor cleft to the surface. Plato 

 presents all the features which are characteristic of its having 

 had such an origin : the floor, some 3800 feet below the general 

 summit of the wall, may be regarded as evidence of its having at 

 some anterior period subsided not a very great depth compared 

 with the diameter (60 miles) ; the general smoothness of the 

 floor may have resulted from the upwelling of fluid material 

 through the central and other orifices; the existence of the 

 " fault " across the floor may testify to its having been shaken 

 and dislocated by an " earthquake," the openings in the wall 



