NoVKMIiBR 1, 1898.1 



KNOW LEDGE 



211 



photognqili in the July number, the long iicl{.e on tbe Mare 

 Trau(iuillitatis stretchiiiy: from near Eoss ((j'i) to near 

 Sabine (05), and tbe ridge on tie Mare Ncctaris stretching 

 northward Irom Beaumont (32'2) ; but it should bo re- 

 membered that low-lying ridges on maria are more likely 

 to be detected if they have a north and south trend than 

 if they run east and west, because the shadows thrown by 

 them as the sun rises will be more conspicuous if the ridge 

 lies nearly parallel to the tiriuinatnr or division between 

 dawn and darlsness upon the moon's surface. 



Probably few readers will be inclined to consider that 

 these inconspicuous ridges are the weathered relics of 

 ancient mountain chains, which mark the lines of crump- 

 ling of the lunar crust, before the moon commenced to 

 turn the same hemisphere constantly towards the earth. 

 But they are worthy of close study, as they seem to diiler 



Fig. >S. — Tlie Mare Serenitiitis. 



in character from the more conspicuous volcanic features 

 to which attention has hitherto been chiefly directed. 



If, as seems probable, these mountain ridges were formed 

 at a later period than the plains on which they stand, 

 and if we may safely follow terrestrial analogies, these 

 lunar ridges ati'ord evidence that tbe moon has sensibly 

 cooled and shrunk since the maria were formed ; and they 

 would lead us to conclude that the maria are probably 

 composed of horizontally stratified rocks capable of being 

 piessed or pushed into folds and ridges. 



Such horizontal stratification does not necessarily 

 imply the deposition of fediment on an ocean bottom, or 

 water action. Horizontal strata might be composed of 

 \olcanic products, such as alternating beds of harder and 

 softer lavas, volcanic mud and scoriiB, but we should 

 expect such beds to be thicker near to the volcanic vents 

 fi om which they issued, forming a more or less steep 

 or gently inclined cone, with possibly a crater and lake of 

 lava over the throat of the volcano. If we conceive of 

 the Mare Imbrium as representing the solidified surface 



of such a vast lake of lava, we must be prepared to con- 

 ceive of the existence of a lunar crater some 000 miles in 

 diameter. But the Mare Nubiura and Oceanus Procellarum 

 are still more difiicult to account for on such a theory, for 

 they have no surrounding walls, and the one plain merges 

 into the other without any line of demarcation. Volcanic 

 mud might possibly have overflowed such an area, but it 

 is difiicult to conceive of a flood of lava hot enough and 

 liquid enough to flow evenly over such an immense region ; 

 and in order to account for horizontal stratification we 

 must conceive of more than one ebbing and flowing of the 

 flood, without leaving a trace of a wall around its margin, 

 or an indication of the position of the vent from which 

 the flood issued. 



Some may feel inclined to assume that these vast plains 

 correspond to the lunar surface which was formed when 

 the moon first solidified from the liquid state, but the 

 circular form of most of the smaller maria. and the fact 

 that some of them are surrounded or partly surrounded 

 by walls, would lead to the conclusion that these level 

 areas are due to some levelling action which has spread 

 from a centre and levelled surrounding inequalities. 



If the lunar rocks are similar to terrestrial rocks and 

 lavas, it seems probable that the original lunar surface 

 would not have solidified in horizontal strata, for, as Prof. 

 G. F. Becker, of the United States Geological Survey, has 

 shown, most lavas and terrestrial rocks expand on melting 

 and contract on solidifying ; so that if a crust formed on a 

 liquid ocean of such material it would soon break up by 

 its own weight and sink, bringing other hot material to 

 the surface, which would again solidify and sink, causing a 

 complete mixture until the whole was solidified,'- and thus, 

 unless subsequent changes took place by water action or 

 otherwise, the original surface would not be stratified. 



The various Hygimis. 



tints of the lunar 

 plains would lead 

 us to conclude 

 that they are not 

 uniformly com- 

 posed of the same 

 material, or that 

 some white or 

 dark material has 

 been distributed 

 over their surface 



in patches. A careful study of the forms of these 

 patches may possibly give us some clue to the action 

 which has been going on. The darker parts of the 

 maria probably correspond in tint to very dark terres- 

 trial rocks, for the moon as a whole reflects less than 

 a quarter (more accurately two-ninths) of the light which 

 would be reflected to us by a sphere of snow. The 

 maria correspond in area to nearly half the area of the 

 visible lunar hemisphere, and the mountainous regions 

 of the moon are not by any means all completely white. 

 So that, assuming the whitest parts of the moon to cor- 

 respond in their light-reflecting power (or albedo) to snow, 

 the whole mountainous area probably reflects about a third 

 of the light which would be reflected by snow, and the 

 darker parts of the maria would probably reflect con- 

 siderably less than a tenth of the light which would be 



Fig. il. 



Qiuie Vapuriim. 



* Prof. Becker account.' for the apparently solid character of the 

 material composing tlie liody of the earth h\ supposing that it is 

 folidilicd hy pressure. Water, which expands on freezing, can by 

 pressure be kept from solidifying, and similarly he concludes that 

 lavas and slags, which expand in melting, can by pressure opposing 

 their expansion be kept in their solid conditiim though at a 

 temperature aboTe their fusing point when not under pressure. 



