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.
Fig. 33.
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.
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.