A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 37 



trial rocks, we have reason to suppose that they are due to faults which skirt the 

 shores and which occurred in the basement rocks while the lava sheet was still in 

 a plastic state. This supposition has its difficulties, for there is no evident reason 

 why such faultings should occur ; faults with vertical displacement are very rare on 

 the surface of the moon, and in no case are they found in any such order as we 

 need to have them to account for the shore wrinkles like those curving around the 

 borders of the maria. 



Less distinct than the typical continuous ridges, but probably to be connected 

 with them, as lesser phenomena of the same order, we have, as before noted, on all 

 the maria and on some of the greater vulcanoids' floors, faint wrinkles of great linear 

 extent. The relation of these to the larger ridges appears to be confirmed by a 

 series in which it is impossible to determine any break. I am therefore disposed 

 to place all the elongate wrinkles in one group, regarding the typical examples 

 hundreds of miles in length as structurally related to the slight, relatively short 

 foldings which are barely revealed by the telescope. On close examination of the 

 more characteristic elongate ridges it appears likely that they are not, as they 

 appear at first sight to be, even arches, but in some instances at least are com- 

 pounded of smaller wrinkles arranged in a more or less parallel order. As these 

 minute features are discernible only by their shadows, it is as yet undetermined 

 whether they are subordinate ridges forming a kind of chain or fractured blocks. 

 I am inclined to think it probable that they are of the last-named nature, for the 

 reason that analogy with terrestrial lavas would indicate that solidified superficial 

 lava would fracture and not fold into arches. Some of these ridges appear to 

 have craterlets on their summits. 



It is also to be noted that, while the systems of low elevation which we are 

 considering have great continuity, there is an evident tendency to break the con- 

 tinuity, so that the chain is composed of separate links, each parted from the other, 

 as in terrestrial mountain chains. Here and there these units are arranged in an 

 echelon order, as is the case in many terrestrial mountain chains such as the Alle- 

 ghanies. This arrangement makes the likeness of these lunar elevations to ter- 

 restrial mountains more evident than any other of its reliefs. 



A third group of lunar elevations, possibly akin to the long ridges above 

 described, is found in the domes which abound in many parts of the surface ; 

 they are, according to my observations, commonest on those parts where vulcan- 

 oids are rare. I have suggested that certain, or perhaps all of them, may be 

 incipient craters. These domes are found on the maria, though here they are of 

 prevailingly smaller size, as well as on the older, more elevated surfaces ; in num- 

 ber they rival the crateriform structures. Following the plan of grouping the 

 lunar features, when possible, into series, I have endeavored so to connect the 

 domes with the elongate arches before described. There are many examples of 

 domes which are somewhat elongate, say with the major axis near twice the 

 extent of the minor, but I have not been able to unite the two groups by any 

 complete series of transitional steps and therefore am led to consider them as 

 possibly distinct. 



