492 
NATURE 
[SEPTEMBER 21, 1899 
that the latter are more often directed tangentially to the 
rampart than in a line with the centre. All these 
circumstances tend to make one consider the little 
orifices situated either on the central mass, on the ridge 
which limits the walled plain, or in the immediate 
neighbourhood, as the real seat of eruptive activity, 
which one might have been tempted to attribute to the 
orifice itself. 
Plate XVIII., which comprises the southern pole, 
gives us a contour deformed by important excrescences. 
The Tycho region shows itself, at the setting as well as 
at the rising of the sun, rich in prominent ridges, which 
serve as limits to the walled plains they meet, and im- 
pose on them polygonal or elongated forms. A more 
attentive examination shows the existence of two super- 
posed systems of parallel ridges, which cut the surface 
up into quadrilaterals. The influence of these align- 
ments has made itself felt, not only in the primitive 
formation of the walled plains, but in the: successive 
annexations which have often constituted a new wall, at 
some distance behind the first, as can be seen in Clavius. 
No trace of these angular fashions are to be found in 
the little parasite orifices, of recent date, which uni- 
formly tend towards the perfect circle. Blancanus, without 
approaching the extent of Clavius, is classed with it by 
its clearness, its great depth, by the indented shadow 
which reproduces inequalities of the crest, and would 
lend itself admirably to measurements of altitude. In 
spite of the great differences of level noted, this region 
is very uniform in tint. This characteristic is due to the 
white colour which the Tycho trails throw over the 
whole. 
Taken from the same cliché as the preceding one, 
Plate XIX. offers quite another aspect. We see plains 
prevailing here, sprinkled with islands and _ brilliant 
craters, furrowed with prominent veins or crevasses, and 
covered in certain parts by large trails which emanate 
from Copernicus and Tycho. We have already become 
familiar with this region in Plate VIII. of the Atlas. The 
comparison of the photographs dealing with very different 
phases is instructive. We note again the relative per- 
manency of the bright areolz, and the periodic variabiliy 
of the dark spots.. The phenomenon of the encroach- 
ment onand of the submersion and final destruction of the 
walled plains can be observed here in all its degrees, 
and we meet with many cases where the depression has 
engulfed a half of the enclosure and the interior plain 
without affecting the rest of the rampart, or even the 
central crater. 
Plate XX. takes us back again to the western hemi- 
sphere, to a part where the relief shows itself with ex- 
treme energy. Numerous local sinkings have here 
reduced the capacity of the crust, without its having to 
submit (to follow the contraction of the liquid centre) to 
a general sinking, accompanied by submersion. Various 
indications prove, however, that a movement of this sort 
has been begun. Thus, the great fracture of the Altai 
mountains, visible near the western edge of the photo- 
graph, skirts at a distance the sea of Nectar, and seems 
to prepare for its extension. 
Another depressed space, also very vast, occupies the 
central part of the photograph, but has not succeeded in 
defining its contour, nor in determining the appearance 
of a sea. Most of the walled plains involved in this 
movement have amongst them a very marked family 
likeness, with a flat bottom and regular rampart. Those 
which have remained outside have kept their primitive 
physiognomy better, and retained in a great number of 
cases their central craters. Apart from these lines of 
circular depression, we see certain rectilinear tracts of 
primitive rocks extending over great stretches. As ele- 
vations they have formed an obstacle to the expansion 
of the walled plains. As depressions they have, on 
the contrary, made it easier, and many of them have 
NO. 1560, VOL. 60] 
transformed themselves into regular chaplets of small 
craters. 
Plate XXI. conducts one still further west, up to the 
illuminated edge of the moon. The characteristics 
already verified in the Mare Humorum reappear in the 
Mare Crisium in a perhaps more accentuated degree ; there 
are rarity of irregularities on the interior plains, elevation 
and regularity of the wall, persistence of a concentric 
terrace remaining adherent to the edge, accumulation of 
dark spots near the periphery. Quite near, the Mare 
Fecunditatis shows, besides its network of prominent 
ridges, large undulations of a rather convex character, 
like those of terrestrialseas. The intermediate plateaus, 
poor in walled plains, seem to be the fairly well pre- 
served testimony of an ancient period. In the neigh- 
bourhood of Taruntius it presents a smooth region, 
probably levelled by an abundant volcanic deposit. Every- 
where else it is furrowed with deep valleys, which tend to 
orient themselves along the meridian, and this direction 
seems to impose itself more and more on approaching 
the illuminated edge. A double system of alignments, 
cutting each other almost at right angles, prevails in the 
Pyrenees, which form the terminator at the upper part of 
the -photograph, and Pétavius reveals itself, as well as 
many other walled plains of the first order, inscribed in 
a quadrilateral. Nearer the equator, Langrenus, with its 
double central mountain, its concentric terraces, its 
divergent trails, affords a quantity of eruptive charac- 
teristics which Copernicus and Tycho perhaps alone 
reunite in the same degree. 
Collected on the next photograph (Pl. XXII.) we find, 
in a very limited space, five remarkable specimens of the 
great crevasses of the crust, that of Sabine, Sosigenes, 
Pliny, Ariadzeus and Hyginus. The first three follow 
more or less the borders of a sea, and may be considered 
as separating a depressed region from the strip which 
has remained adherent to the mountainous plateau. The 
fissure of Ariadzeus, prolonged a great stretch without 
regard to the relief of the surface, cutting many trans- 
versal chains, appears to date from an epoch when the 
crust was still disjoined and mobile in the tangential 
direction. 
Hyginus presents, besides, quite a series of, circular 
enlargements, which transform, as it were, a crevasse into 
a chaplet of craters. 
The plain which surrounds Arago contains two 
characteristic examples of formations extremely rare at 
the present time. They are vast intumescences, 15 km. 
to 20 km. in size, in which the sinking of the central part 
would give rise to the ordinary physiognomy of the 
walled plains. 
The last sheet may be recommended as illustrating 
well the structure of the mountainous masses of the moon, 
saved by some means, and left in relief after the form- 
ation of the seas. 
Draughtsmen have had to content themselves here, 
in presence of the multitude of details, and of their 
variability of aspect, with a conventional figuration, 
where few objects, except those which form projections, 
could be named or identified. Our photograph renders 
a much more precise topographic description possible. 
The most peaked part of the Apennines and the Alps 
show a number of summits which can be recognised 
on the sheets of the preceding Parts, in spite of the 
change of incidence of the light. We see a character- 
istic appearing, noted by geographers as special to 
chains of the most recently elevated mountains, where 
the erosion has not had time to destroy the primitive 
constitution ; it is a. marked dissymmetry in the relief, 
throwing the highest summits to one side, and dividing 
the mass into two parts of very unequal average 
slopes. 
So much for the descriptive matter. We now come to 
