PLATE XV.
MERCATOR & CAMPANUS.
We remark, too, that there is a prevailing tendency of the ranges just mentioned to present their loftiest constituents in abrupt terminal lines, facing nearly the same direction, the reverse of that towards which they are carried by the moon’s rotation; and as they recede from the high terminal line, the mountains gradually fall off in height, so that in bulk the ranges present the “crag and tail” contour which individual hills upon the earth so frequently exhibit.
Isolated peaks are found in small numbers upon the moon; there are a few striking examples of them nevertheless, and these are chiefly situated in the mountainous region just alluded to. Several are seen to the east (right hand) of the Alpine range depicted on [Plate XIV]. The best known of these is Pico, which rises abruptly from a generally smooth plain to a height of 7000 feet. It may be recognized as the lower of the two long shadowing spots located almost centrally above the crater Plato in the illustration just mentioned. Above it, at an actual distance of 40 miles, there is another peak (unnamed) about 4000 feet high; and away to the west, beyond the small crater joined by a hill-ridge to Plato, is a third pyramidal mountain nearly as high as Pico.
It seems natural to regard the great mountain chains as agglomerations of those peaks of which we have isolated examples in Pico and its compeers, and thus to consider that the formation of a mountain chain has been a multiplication of the process that formed the single pyramid-shaped eminences. At first thought it might appear that the great mountain ranges were produced by bodily upthrustings of the crust of the moon by some subsurface convulsions. But such an explanation could hardly hold in relation to the isolated peaks, for it is difficult, if not impossible, to conceive that these abrupt mountains, almost resembling a sugarloaf in steepness, could have been protruded en masse through a smooth region of the crust. On the contrary it is quite consistent with probability to suppose that they were built up by a slow process somewhat analogous to that to which we have ascribed the piling of the central cones of the great craters. We believe they may be regarded as true mountains of exudation, produced by the comparatively gentle oozing of lava from a small orifice and its solidification around it; the vent however remaining open and the summit or discharging orifice continually rising with the growth of the mountain, as indicated in the annexed cut, [Fig. 36]. This process is well exemplified in the case of a water fountain playing during a severe frost; the water as it falls around the lips of the orifice freezes into a hillock of ice, through the centre of which, however, a vent for the fluid is preserved. As the water trickles over the mound it is piled higher and higher by accumulating layers of ice, till at length a massive cone is formed whose height will be determined by the force or “head” of the water. Substitute lava for water and we have at once a formative process which may very fairly be considered as that which has given rise to the isolated mountains of the moon.
Fig. 36.
Fig. 37.
