ON THE GREAT RING-FORMATIONS NOT MANIFESTLY VOLCANIC.
In our previous chapter we have given a reason for regarding as true
volcanic craters all those circular formations, of whatever size, that
exhibit that distinctive feature _the central cone_. Between the
smallest crater with a cone that we can detect under the best telescopic
conditions, namely, the companion to Hell, 1¾ mile diameter, and the
great one called Petavius, 78 miles in diameter, we find no break in the
continuity of the crater-cum-cone system that would justify us in saying
that on the one side the volcanic or eruptive cause ceased, and on the
other side some other causative action began. But there are numerous
circular formations that surpass the magnitude of Petavius and its
peers, but that have no central cone, and are, therefore, not so
manifestly volcanic as those which possess this feature. Our map will
show many striking examples of this class at a glance. We may in
particular refer _inter alia_ to Ptolemy near the centre of the moon, to
Grimaldi (No. 125), Shickard (No. 28), Schiller (No. 24), and Clavius
(No. 13), all of which exceed 100 miles in diameter. Even the great
_Mare Crisium_, nearly 300 miles in diameter, appears to be a formation
not distinct from those which we have just named. These present little
of the generic crater character in their appearance; and they have been
distinguished therefrom by the name of _Walled_ or _Ramparted Plains_.
Their actual origin is beyond our explanation, and in attempting to
account for them we must perforce allow considerable freedom to
conjecture. They certainly, as Hooke suggested, present a “broken
bubble”-like aspect; but one cannot reasonably imagine the existence of
any form of mineral matter that would sustain itself in bubble form over
areas of many hundreds of square miles. And if it were reasonable to
suppose the great rings to be the foundations of such vast volcanic
domes, we must conclude these to have broken when they could no longer
sustain themselves, and in that case the surface beneath should be
strewed with _débris_, of which, however, we can find no trace.
Moreover, we might fairly expect that some of the smaller domes would
have remained standing: we need hardly say that nothing of the kind
exists.
[Illustration: Fig. 32.]
The true circularity of these objects appears at first view a remarkable
feature. But it ceases to be so if we suppose them to have been produced
by some very concentrated sublunar force of an upheaving nature, and if
only we admit the homogeneity of the moon’s crust. For if the crust be
homogeneous, then _any_ upheaving force, deeply seated beneath it, will
exert itself _with equal effects at equal distances from the source_:
the lines of equal effect will obviously be radii of a sphere with the
source of the disturbance for its centre, and they will meet a surface
over the source in a circle. This will be evident from Fig. 32, in which
a force is supposed to act at F below the surface s s s s. The matter
composing s s being homogeneous, the action of F will be equal at equal
distances in all directions. The lines of equal force, F_ f_, F _f_,
will be of equal length, and they will form, so to speak, radii of a
sphere of force. This sphere is cut by the plane at _s s s s_, and as
the intersection necessarily takes place everywhere at the extremity of
these radii, the figure of intersection is demonstrably a circle (shown
in perspective as an ellipse in the figure). Thus we see that an intense
but extremely confined explosion, for instance, beneath the moon’s crust
must disturb a _circular_ area of its surface, if the intervening
material be homogeneous. If this be not homogeneous there would be,
where it offered _less_ than the average resistance to the disturbance,
an outward distortion of the circle; and an opposite interruption to
circularity if it offers _more_ than the average resistance. This
assumed homogeneity may possibly be the explanation of the general
circularity of the lunar surface features, small and great.
[Illustration: Fig. 33.]
[Illustration: Fig. 34.]
We confess to a difficulty in accounting for such a very local
generation of a deep-seated force; and, granting its occurrence, we are
unprepared with a satisfactory theory to explain the resultant effect of
such a force in producing a raised ring at the limit of the circular
disturbance. We may indeed, suppose that a vast circular cake or conical
frustra would be temporarily upraised as in Fig. 33, and that upon its
subsidence a certain extrusion of subsurface matter would occur around
the line or zone of rupture as in Fig. 34. This supposition, however,
implies such a peculiarly cohesive condition of the matter of the
uplifted cake, that it is doubtful whether it can be considered tenable.
We should expect any ordinary form of rocky matter subjected to such an
upheaval to be fractured and distorted, especially when the original
disturbing force is greater in the centre than at the edge, as,
according to the above hypothesis, it would be; and in subsiding, the
rocky plateau would thus retain some traces of its disturbance; but in
the circular areas upon the moon there is nothing to indicate that they
have been subjected to such dislocations.
[Illustration: Fig. 35. A A. Fissures gaping downwards and injected
by intumescent lava beneath. B B B. Fissures gaping upwards and
allowing wedges of rock to drop below the level of the intervening
masses, C C. Wedges forced upwards by horizontal compression. E F.
Neutral plane or pivot axis, above and below which the directions of
the tearing strain and horizontal compression are severally
indicated by the smaller arrows; the larger arrows beneath represent
the direction of the primary expansive force.]
Mr. Scrope in his work on volcanoes has given a hypothetical section of
a portion of the earth’s crust, which presents a bulging or tumescent
surface in some measure resembling the effect which such a cause as we
have been considering would produce. We give a slightly modified version
of his sketch in Fig. 35, showing what would be the probable phenomena
attending such an upheaval as regards the behaviour of the disturbed
portion of the crust, and also that of the lava or semifluid matter
beneath: and, as will be seen by the sketch, a possible phase of the
phenomena is the production of an elevated ridge or rampart at the
points of disruption _c c_; and where there is a ring of disruption, as
by our hypothesis there would be, the ridge or rampart _c c_ would be a
circle. In this drawing we see the cracking and distortion to which the
elevated area would be subjected, but of which, as previously remarked,
the circular areas of the moon present no trace of residual appearance.
[Illustration: PLATE XIV.
PLATO.]
Those who have offered other explanations of these vast ring-formed
mountain ranges, have been no more happy in their conjectures. M. Rozet,
who communicated a paper on selenology to the French Academy in 1846,
put forth the following theory. He argued that during the formation of
the solid scoriaceous pelicules of the moon, circular or tourbillonic
movements were set up; and these, by throwing the scoria from the centre
to the circumference, caused an accumulation thereof at the limit of the
circulation. He considered that this phenomenon continued during the
whole process of solidification, but that the amplitude of the whirlpool
diminished with the decreasing fluidity of the surface material.
Further, he suggested that when many vortices were formed, and the
distances of their centres, taken two and two, were less than the sums
of their radii, there resulted closed spaces terminated by arcs of
circles; and when for any two centres the distance was greater than the
sum of the radii of action, two separate and complete rings were formed.
We have only to remark on this, that we are at a loss to account for the
origination of such vorticose movements, and M. Rozet is silent on the
point. If the great circles are to be referred to an original sea of
molten matter, it appears to us more feasible to consider that wherever
we see one of them there has been, at the centre of the ring, a great
outflow of lava that has flooded the surrounding surface. Then, if from
any cause, and it is not difficult to assign one, the outflow became
intermittent, or spasmodic, or subject to sudden impulses, concentric
waves would be propagated over the pool and would throw up the scoria or
the solidifying lava in a circular bank at the limit of the fluid area.
This hypothesis does not differ greatly from the _ebullition_ theory
proposed by Professor Dana, the American geologist, to explain these
formations. He considered that the lunar ring-mountains were formed by
an action analogous to that which is exemplified on the earth in the
crater of Kilauea, in the Hawaiian islands. This crater is a large open
pit exceeding three miles in its longer diameter, and nearly a thousand
feet deep. It has clear bluff walls round a greater part of its circuit,
with an inner ledge or plain at their base, raised 340 feet above the
bottom. This bottom is a plain of solid lavas, entirely open to day,
which may be traversed with safety (we are quoting Professor Dana’s own
statement written in 1846, and therefore not correctly applying to the
present time): over it there are pools of boiling lava in active
ebullition, and one is more than a thousand feet in diameter. There are
also cones at times, from a few yards to two or three thousand feet in
diameter, and varying greatly in angle of inclination. The largest of
these cones have a circular pit or crater at the summit. The great pit
itself is oblong, owing to its situation on a fissure, but the lakes
upon its bottom are round, and in them, says Professor Dana, “the
circular or slightly elliptical form of the moon’s craters is
exemplified to perfection.”
Now Dana refers this great pit crater and its contained lava-lakes to
“the fact that the action at Kilauea is simply _boiling_, owing to the
extreme fluidity of the lavas. The gases or vapours which produce the
state of active ebullition escape freely in small bubbles, with little
commotion, like jets over boiling water; while at Vesuvius and other
like cones they collect in immense bubbles before they accumulate force
enough to make their way through; and consequently the lavas in the
latter case are ejected with so much violence that they rise to a height
often of many thousand feet and fall around in cinders. This action
builds up the pointed mountain, while the simple boiling of Kilauea
makes no cinders and no cinder cones.”
Professor Dana continues, “If the fluidity of lavas, then, is sufficient
for this active ebullition, we may have boiling going on over an area of
an indefinite extent; for the size of a boiling lake can have no limits
except such as may arise from a deficiency of heat. The size of the
lunar craters is therefore no mystery. Neither is their circular form
difficult of explanation; for a boiling pool necessarily, by its own
action, extends itself circularly around its centre. The combination of
many circles, and the large sea-like areas are as readily
understood.”[10]
In justice to Professor Dana it should be stated that he included in
this theory of formation all lunar craters, even those of small size and
possessing central cones; and he put forth his views in opposition to
the eruptive theory which we have set forth, and which was briefly given
to the world more than twenty-five years ago. As regards the smallest
craters with cones, we believe few geologists will refuse their
compliance with the supposition that they were formed as our
crater-bearing volcanoes were formed: and we have pointed out the
logical impossibility of assigning any limit of size beyond which the
eruptive action could not be said to hold good, so long as the central
cone is present. But when we come to ring-mountains having no cones, and
of such enormous size that we are compelled to hesitate in ascribing
them to ejective action, we are obliged to face the possibility of some
other causation. And, failing an explanation of our own that satisfied
us, we have alluded to the few hypotheses proffered by others, and of
these Professor Dana’s appears the most rational, since it is based upon
a parallel found on the earth. In citing it, however, we do necessarily
not endorse it.