A brief general description of the phænomena of the Moon will prepare us for an examination of its topography. We have then

1. The Grey Plains, or Seas as they were formerly believed to be, and are still termed for convenience.* These are evidently dry flats-if the term 'flat' can be applied to surfaces shewing visibly the convexity of the globe-analogous to the deserts and prairies and pampas of the Earth. B. and M. find that they do not form portions of the same sphere, some lying deeper than others: they are usually of a darker hue than the elevated regions which bound them, but, with a strong general resemblance, each has frequently some peculiar characteristic of its own.

2. The Mountain Chains, Hills, and Rulges. These are of very various kinds: some are of vast continuous height and extent, some flattened into plateaus intersected by ravines, some rough with crowds of hillocks, some sharpened into detached and precipitous peaks. The common feature of the mountainchains on the Earth, a greater steepness along one side, is very perceptible here, as though the strata had been tilted in a similar manner. Detached masses and solitary pyramids are scattered here and there upon the plains, frequently of a height and abruptness paralleled only in the most craggy regions of

The varieties of colour, if not arising from vegetation, may indicate an amount of weathering' which must be great from the distance at which it is perceptible: nor can we confidently affirm that with powerful telescopes no traces can be detected of the hand of time.

*

Riccioli, when he recast the lunar nomenclature, and substituted the names of philosophers for the feeble geographical analogies of Hevel, retained the generic title of 'seas,' though he altered their designations. The reform attempted by G., who would have had them called 'surfaces,' has never taken effect.

the Earth.* Every gradation of cliff and ridge and hillock succeeds: among them a large number of narrow banks of slight elevation but surprising length,† extending for vast distances through level surfaces; these so frequently form lines of communication between more important objects,-uniting distant craters or mountains, and crowned at intervals by insulated hills, that Schr. formerly, and B. and M. in modern times, have ascribed them to the horizontal working of an elastic force, which, when it reached a weaker portion of the surface, issued forth in a vertical upheaval or explosion. The fact of the communication is more obvious than the probability of the explanation.

:

3. The Crater-Mountains, comprising both the ridge and the included cavity. These are the grand peculiarities of the Moon commonly, and probably with correctness, ascribed to volcanic agency: yet differing in several respects from the foci of eruption on our own globe: on the Earth, they are usually openings on the summits or sides of mountains-on the Moon, depressions below the adjacent surface, even when it is a plain or valley; on the Earth, the mass of the cone usually far exceeds the capacity of the crater-on the Moon, they are much nearer equality; on the Earth, they are commonly the sources of long lava-streams- -on the Moon, traces of such outpourings are rare; on the Earth, their dimensions are comparatively inconsiderable on the Moon, many of them are, the

* Schm. ascribes less rapidity to the gradients than is here supposed. Phillips, however, says, 'the steepness of the moon-crater walls and slopes is much greater in general than in any, except very rare examples, known among the volcanic regions of the earth.'-Phil. Trans. 1868.

† Schr. gives a length of 630 or 640 miles to a ridge connecting the spots Copernicus and Kirch.

grey plains excepted, among the largest of its features. When, however, allowance has been made for the inferior power of gravity on the Moon, through which a six-fold displacement in height or distance would be caused by the same amount of force,-for the possible difference of materials, and for the more rapid cooling produced by radiation in the absence of an atmosphere, it is quite conceivable that volcanic force, similar to that on the Earth, may have been the real agent, though in a greatly modified form. Any one may see, with the ingenious Hooke, a strong resemblance to the rings left by gaseous bubbles; but to this impression mechanical difficulties arising from the cohesion of materials have been opposed, and a more consistent explanation sought in the idea that the larger craters may be the remains of molten lakes; in these, left for a while unfrozen in the general cooling and crusting over of the oncefiery globe, an alternate shrinking and overflowing of lava, from a fluctuating pressure from beneath, might gradually produce the existing forms. We have nothing on a corresponding scale on the Earth; but the craters of the Sandwich Islands, Kirauea and Haleakala, the one a fused, the other a frozen, lake of lava, with the small 'blowing-cones' which eject only cinders and ashes, afford an analogy,* the striking nature of which will be apparent from the following representation of the latter, taken from a view in Elwes's Sketcher's Tour.'t Difficulties no doubt remain, but we can hardly

*The craters of Java are also said to bear out this comparison.

† Haleakala, the house of the Sun,' in E. Maui, is of an oblong form, with two great openings in the wall, more than 30 miles in circumference, 10,000 feet above the sea, and about 2,000 feet deep. On the floor are 12 or 13 small red or yellow cones. The highest summits in the view are the two snowy volcanos of Hawaii, seen over the clouds in a very faint distance: our sketch gives only the general effect.

wonder at them, while geologists are still so little agreed about 'elevation-craters' and submarine volcanos on our own globe. The circular cavities of the Moon are arranged in three classes, Walled (or Bulwark) Plains, Ring-Mountains, and

Craters: a fourth includes little pits without, or with scarcely a visible ring. The second and third differ chiefly in size; but the first have a character of their own;-the perfect resemblance of their interiors to the grey plains, as though they had been originally deeper, but filled in subsequently with the same material; many of them in fact bearing evident marks of having been broken down and overflowed from the outside. Their colour is often suggestive of some kind of vegetation, though it is difficult to reconcile this with the apparent deficiency of air and water. It has been ingeniously suggested that a low stratum of carbonic acid gas-the frequent product of volcanos, and long surviving their activity *-may in such * For instance, among the ancient craters of Auvergne, where it exists in great quantity.

situations support the life of some kind of plants: and the idea deserves to be borne in mind in studying the changes of relative brightness in some of these spots.* The deeper are usually the more concave craters; but the bottom is often flat, sometimes convex; and frequently shews subsequent disturbance, in ridges, hillocks, minute craters, or more generally, as the last effort of the eruption, central hills of various heights, but seldom attaining that of the wall, or even, according to Schm., the external level. The ring is usually steepest within (as in terrestrial craters),† and many times built up in vast terraces, frequently lying, Schm. says, in pairs divided by narrow ravines. Nasmyth refers these-not very probably-to successively decreasing explosions; in other cases he more reasonably ascribes them to the slipping down of materials upheaved too steeply to stand, and undermined by lava at their base, leaving visible breaches in the wall above: they would be well explained on the supposition just mentioned of fluctuating levels in a molten surface. Small transverse ridges óccasionally descend from the ring-chiefly on the outside: great peaks often spring up like towers upon the wall; gateways at times break through the rampart, and in some cases are multiplied till the remaining piers of wall resemble the stones of a huge megalithic circle. A succession of eruptions may be constantly traced, in the repeated encroachment of rings on each other, where, as Schm. says, the ejected mate

* G. thought he perceived, in the grey tints of depressed surfaces, some of which vary with the amount of solar light, traces of several kinds of vegetation, comprised between 65° N. and 55° S. latitude, and preserving the correspondence observed on the Earth between increasing latitude and elevation.

Schm. gives 1° to 4°, and 20° to 50°, as the respective average inclinations without and within (the former, under-rated ?).