The Moon | Page 6

Thomas Gwyn Elger
south, throwing
out many minor branches, and ultimately dies out under a great rocky
promontory--the Promontory Acherusia, at the western termination of
the Haemus range. A comparatively low power serves to show the
curious structural character of this immense ridge, which appears to
consist of a number of corrugations and folds massed together, rising in
places, according to Neison, to a height of 700 feet and more. The Mare
Imbrium also affords an example of a ridge, which, though shorter, is
nearly as prominent, in that which runs from the bright little ring-plain
Piazzi Smyth towards the west side of Plato. The region round

Timocharis and other quarters of the Mare are likewise traversed by
very noteworthy features of a similar class. The Oceanus Procellarum
also presents good instances of ridges in the marvellous ramifications
round Encke, Kepler, and Marius, and in the region north of
Aristarchus and Herodotus. Perhaps the most perfect examples of
surface swellings are those in the Mare Tranquilitatis, a little east of the
ring-plain Arago, where there are two nearly equal circular mounds, at
least ten miles in diameter, resembling tumuli seen from above. Similar,
but more irregular, objects of a like kind are very plentiful in many
other quarters.
It is a suggestive peculiarity of many of the lunar ridges, both on the
Maria and elsewhere, that they are very generally found in association
with craters of every size. Illustrations of this fact occur almost
everywhere. Frequently small craters are found on the summits of these
elevations, but more often on their flanks and near their base. Where a
ridge suddenly changes its direction, a crater of some prominence
generally marks the point, often forming a node, or crossing-place of
other ridges, which thus appear to radiate from it as a centre.
Sometimes they intrude within the smaller ring-mountains, passing
through gaps in their walls as, for example, in the cases of Madler,
Lassell, &c. Various hypotheses have been advanced to account for
them. The late Professor Phillips, the geologist, who devoted much
attention to the telescopic examination of the physical features of the
moon, compared the lunar ridges to long, low, undulating mounds, of
somewhat doubtful origin, called "kames" in Scotland, and "eskers" in
Ireland, where on the low central plain they are commonly found in the
form of extended banks (mainly of gravel), with more or less steep
sides, rising to heights of from 20 to 70 feet. They are sometimes only a
few yards wide at the top, while in other places they spread out into
large humps, having circular or oval cavities on their summits, 50 or 60
yards across, and as much as 40 feet deep. Like the lunar ridges, they
throw out branches and exhibit many breaches of continuity. By some
geologists they are supposed to represent old submarine banks formed
by tidal currents, like harbour bars, and by others to be glacial deposits;
in either case, to be either directly or indirectly due to alluvial action.
Their outward resemblance to some of the ridges on the moon is

unquestionable; and if we could believe that the Maria, as we now see
them, are dried-up sea-beds, it might be concluded that these ridges had
a similar origin; but their close connection with centres of volcanic
disturbance, and the numbers of little craters on or near their track,
point to the supposition that they consist rather of material exuded from
long-extending fissures in the crust of the "seas," and in other surfaces
where they are superimposed. This conjecture is rendered still more
probable by the fact that we sometimes find the direction of clefts
(which are undoubted surface cracks) prolonged in the form of long
narrow ridges or of rows of little hillocks. We are, however, not bound
to assume that all the manifold corrugations observed on the lunar
plains are due to one and the same cause; indeed, it is clear that some
are merely the outward indications of sudden drops in the surface, as in
the case of the ridges round the western margin of the Mare Nectaris,
and in other situations, where subsidence is manifested by features
assuming the outward aspect of ordinary ridges, but which are in reality
of a very different structural character.
The Maria, like almost every other part of the visible surface, abound in
craters of a minute type, which are scattered here and there without any
apparent law or ascertained principle of arrangement. Seeing how
imperfect is our acquaintance with even the larger objects of this class,
it is rash to insist on the antiquity or permanence of such diminutive
objects, or to dogmatise about the cessation of lunar activity in
connection with features where the volcanic history of our globe, if it is
of any value as an analogue, teaches us it
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