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



ORIGIN OF LUNAR MOUNTAINOUS RELIEFS. 



As regards the origin of the first-described groups of lunar reliefs, those 

 which form the massive elevated mountains, it may be said that they cannot be 

 placed in the category of terrestrial structures due to folding and faulting com- 

 bined with aqueous erosion. If there be any one certain fact concerning lunar 

 topography it is that it nowhere exhibits the results of water erosion. If oro- 

 genic action such as operates on the earth has acted on the moon, as it may 

 have done in the case of the elongate ridges of the maria, it could give us no 

 more than arches and the fractures incident on their formation. It could not 

 possibly have developed the steep, lofty, and extremely serrate structures such as 

 are found in the greater fields of the so-called mountains. So far as geology 

 enables us to interpret them, these elevations must be due to the ejection of 

 exceedingly viscous lavas, forming heaps such as we have in certain masses of 

 trachytic rocks on the earth. That such ejections do occur on the moon is well 

 shown by the very numerous and often high peaks which have evidently been 

 thrust up in the central part of the lava field enclosed by the greater vulcanoids. 

 In character of summits and slopes these tumefactions of the ring plains are to 

 my seeing essentially like the so-called mountains. They often attain to near 

 the average height of the peaks in the Alps or the Apennines or other lunar 

 fields of crowded elevations. The facts have led me to the following considera- 

 tions and to a working hypothesis based on them : 



Noting that the peaks formed in the central part of the lava floors of the 

 greater vulcanoids clearly indicate that, after a period when tolerably fluid lavas 

 existed beneath the crust, there came a time when these lavas were so viscous 

 that while they might be extruded they would not flow, but retained the shape 

 in which they were spewed out ; noting also that the evidence from the invasion 

 of the vulcanoids by mountain ridges indicates that these elevations were 

 among the more recently formed structures of the maria, we are led to the 

 suggestion that they represent a stage of the eruption when the ejected materials 

 were so viscous that they could no longer form vulcanoids, but poured forth 

 masses which not only did not flow but heaped up near the vent, just as they evi- 

 dently did in the central field of many craters. It is true that small craters are 

 here and there, though rarely, found amid these mountainous elevations ; they 

 may represent the localized remnants of the once general fluid state, remnants 

 sufficient to produce slight eruptions of the earlier type. 



I have already called attention to the fact that the distribution of the ex- 

 ceedingly numerous small bleb-like domes on the lunar surface suggests that 

 they are the first stage in the development of craters, the imprisoned vapors 

 serving to lift the surface although it was not broken through. It appears to 

 me likely that it is in such elevations that we have also the beginnings of the other 

 group of vulcanoids, the ejected peaks. In several parts of the moon, notably 

 in the region where mountainous elevations occur, these domes abound. In 

 some cases small craters occur in the same field, which suggests, as before noted, 



