THE MOOH. 617 
tlie walled concavities^ lower tlian the surrounding parts^ but 
Mr. ISTasmyth has detected a circular wailed plain, in which 
the floor is on the same level as the outer walls, as if the lava 
had bubbled up and filled the enclosure. It may be probable 
that this latter event may have taken place in the case of 
other smooth plains, as in the interior of Plato Archimedes . 
In a fevr cases there appears to have been an upheaving at 
the centre, without any eruption ; and this is very apparent in 
the mountain dferseujze, the floor of which is singularly convex. 
Schroeter has calculated that the cubic contents of the sur- 
rounding* wall are nearly equal to that of the vacant space in 
the interior which dips below the exterior surface ; and 
this applies to both annular concavities and the walled plains. 
In one small crater the difl*erence between the volumes was 
ouly e^^th, in that of Peinliold the difference amounted to as much 
as Jth, whilst in that of Euler the volume of the cavity is double 
that of the wall ; but hundreds of other cavities have no wall 
at all. The peaks at the centre of the walled plains, as in 
those of the annular concavities, are never so hio-h as the 
surrounding wall. They may be found of from fom* to five 
thousand feet high in Maretus, Tijclio, Petavius, and Tlieo- 
pliilus ; but in those cases the outer wall is from three to four 
times that height. It is very singular to see chains of circular 
mountains running* in almost straight lines for great distances. 
Thus, between Hipijctrclms and Ptolemy there is a row 36 
degrees long, and there are two similar rows, one 60 degrees east 
and the other 60 degrees west of the above. The term circu- 
lar is sometimes very inapplicable, as the mountain Egede 
is almost square, and Descartes is 37 miles long and only two 
in breadth. We now come to the smallest of these circular 
formations, viz., those simple depressions in the soil called 
craters, and which may be considered the most numerous of 
all. They occur everywhere — on the summits of the highest 
mountain- chains, in the most depressed portions of the con- 
cavities and walled plains ; they are plentiful in the seas, 
and even can be seen, like parasites, attached to each other. 
Yet the smallest is almost equal to Etna, which is the 
greatest volcano on the earth. As in the case of the walled 
plains, they run together, when a sort of a door joins 
them together. As an example of this, we may take the 
part between Eratosthenes and Copernicus, the landscape 
about Sasserides and Orontius, the parts surroundiug 
Cag)ella and Gensorinus, in the wall and the plain of Alha- 
tegnius. On the north-west of Ptolemy there are six small 
craters, each of from 4,000 to 6,000 feet in diameter, which 
run together in a straight line, and which, as Maedlcr 
observes, present a charming* appearance. Sometimes those 
