an 80
miles in length, and varies in breadth from 6 miles on the S. to less
than 4 miles on the N., where it approaches the S. border of the Mare
Frigoris. For a greater part of its extent it is bounded on the S.W. side
by a precipitous linear cliff, which, under a low evening sun, is seen to
be fringed by a row of bright little hills. These are traceable up to one
of the great mountain masses of the Alps, forming the S.W. side of the
great oval-shaped expansion of the valley, whose shape has been
appropriately compared to that of a Florence oil-flask, and which Webb
terms "a grand amphitheatre." On the opposite or N.E. side, the boundary
of the valley is less regular, following a more or less undulating line
up to a point opposite, and a little N. of, the great mountain mass,
where it abuts on a shallow _quasi_ enclosure with lofty walls, which,
projecting westwards, considerably diminish the width of the valley.
South of this lies another curved mountain ring, which still farther
narrows it. This curtailment in width represents the neck of the flask,
and is apparently about 16 or 17 miles in length, and from 3 to 4 miles
in breadth, forming a gorge, bordered on the W. by nearly vertical
cliffs, towering thousands of feet above the bottom of the valley; and on
the E. by many peaked mountains of still greater altitude. At the
entrance to the "amphitheatre," the actual distance between the colossal
rocks which flank the defile is certainly not much more than 2 miles.
From this standpoint the view across the level interior of the elliptical
plain would be of extraordinary magnificence. Towards the S., but more
than 12 miles distant, the outlook of an observer would be limited by
some of the loftiest peaks of the Alps, whose flanks form the boundary of
the enclosure, through which, however, by at least three narrow passes he
might perchance get a glimpse of the Mare Imbrium beyond. The broadest of
these aligns with the axis of the valley. It is hardly more than a mile
wide at its commencement on the S. border of the "amphitheatre," but
expands rapidly into a trumpet-shaped gorge, flanked on either side by
the towering heights of the Alps as it opens out on to the Mare. The
bottom, both of the "amphitheatre" and of the long wedge-shaped valley,
appears to be perfectly level, and, as regards the central portion of the
latter, without visible detail. Under morning illumination I have,
however, frequently seen something resembli