ON MAPPING THE SURFACE OF THE MOON. 17 
yiz. a kind of “ circumvallation”’ (consisting either of low mountains or, at 
some parts, of depressions) surrounds it, at an average distance from the 
centre of 35':24, not very unlike the second rampart of Iheticus (see letter- 
press, areas IV A*, LV AS, p. 12, and Brit. Assoc. Report, 1866, p. 248), or 
the cliffs surrounding IV A*¢ (see ibid. pp. 18, 249). A portion of a similar 
circumvallation exists on the N.W. of Agrippa. The circumvallation sur- 
rounding IV Af? is best traced on the N. in the mountains W. and E. of 
Godin, including the 8. border of that crater. The depression IV A? 4 and 
the mountain LV A& “ are situated upon this line of circumyallation. Here 
it becomes obscure, but can be traced passing between LV Af 4 and LV Af %, 
and continued in the mountain-chain extending from the junction of Ptole- 
meus and Alphonsus towards Sabine and Ritter (see ante, p. 13). From this 
mountain-chain the curve is continued to its junction with the 8. of Godin. 
The area enclosed by this ‘ circumvallation ” is about equal to that of the 
floor of Ptolemeus. Is this the remnant of a large walled plain which in- 
cluded a crater, nearly central, within it? Its position with regard to Ptole- 
meus is interesting, especially as the diverse groups on and near Hipparchus 
intervene. Its ancient character may be inferred from its being traversed by 
more than one “ fault :’—first, a “ray from Tycho,” which passes across its 
eastern segment; second, by the “fault” IV A* #2, which also crosses this 
segment. The “fault” IV A”* LV A®® orazes the E. edge of the ancient wall. 
The following passage from the Rev. T. W. Webb’s notice of Chacornac’s 
‘Theory of Lunar Physics’ is so much to_the purpose that I quote it at length. 
«*‘ Among other characteristics of the primitive surface, we notice immense 
rings, whose crests alone project above the surrounding plain by some hundreds 
of yards—circular ramparts, the last visible vestiges of great buried craters ; 
and these are cut through by considerable breaches, which permit us to follow 
the level of the maritime soil, where it penetrates their interiors, and to re- 
mark the absence of the slightest difference in surface or structure.’’—Jntel- 
lectual Observer, No. xlvii. Dec. 1865, p. 373. 
The surface of the area included within the line of circumvallation is in 
striking contrast with that of Ptolemeus. If it has ever been in the same 
state as that magnificent plain, the changes it has undergone must have been 
considerable. From a smooth level surface, surrounded by a rampart of 
mountains, the remains of which we are now only able to trace imperfectly, 
it must have passed into a state during which the central parts have been 
elevated, and the surface attained a degree of subdued irregularity very diffe- 
rent from the surfaces of the great walled plains. While nearly every vestige 
of the characteristics of a walled plain has disappeared, the surface included 
by the circumvallation possesses a certain uniformity of aspect which gives an 
individuality to it, and which clearly separates it from the features external 
to it, It is singularly free from craters, two only of any magnitude being 
found upon it, viz. [V A?3, and the crater 8.S.E. of Godin, I A®5. Should 
the conjecture be correct that we have here the remains of a large walled 
plain, the intermediate changes that it has undergone indicate it to have 
been very ancient, perhaps among the most ancient of lunar forms. 
Hipparchus may be regarded as intermediate in character between this for- 
mation and Ptolemeus. We shall have to direct attention (see post, p. 37) to 
the probability that at some anterior epoch the depressed floor of Hipparchus 
and the higher land ‘W. of it were at the same level, and that the whole of 
the district W. of Hipparchus, including the plain under consideration, has 
since been elevated. If so, from the appearance which this walled plain now 
presents, it is probable that the rampart was nearly filled prior to the general 
