146 



KNOWLEDGE. 



[August 1, 1891. 



once continuous are left at very different levels on opposite 

 sides of the fissure. Hence the term " fault." Some of 

 the "faults'" i-cnowu to geologists are not only of great 

 horizontal length as traced along the surface, hut of very 

 considerable depth, and have produced enormous displace- 

 ments. Thus the great Pennine " fault " of the north of 

 England is known to be at least 55 miles long, and has a 

 " throw " of 6-7000 feet, i.e., the rocks on either side have 

 been displaced to that extent. It was probably formed at 

 some time during the upheaval of the Pennine range of 

 hills, which runs north and south as the " fault " also 

 does. The Tyndale " fault " has a throw, of nearly 

 3000 feet, and it rims eastwards for about 50 miles. 

 Fractures not unfrequently occur along the axes of great 

 folds, such as we find in mountain chains, the strata 

 having snapped under the great tension to which they 

 were subjected during upheaval. Thus we find " faults " 

 running parallel with some of the great mountain chains 

 of the world ; the Alps and Himalayas are cases in point. 



This connection between great terrestrial cracks and 

 important mountain ranges is only what might have been 

 expected. The Unita Mountains of Wyoming and Utah 

 consist of one broad flattened fold, with a displacement, 

 m places where the uplift has been greatest, of 20,000 

 feet ! If the lunar streaks under consideration are due to 

 " faults," it is difficult to understand how the level on 

 each side should be so little disturbed. As a general rule, 

 the brightness of the lunar surface corresponds to the 

 altitude of the ground. Mr. Eanyard says the rays do 

 not correspond to lofty ridges, or even to ridges a few 

 hundred feet in altitude, for no ridges casting shadows as 

 the sun rises and sets can be detected as coincident with 

 the streaks. It seems generally admitted that they do not 

 correspond to lava-streams, for the rays run across 

 mountains and plains, and even through the rings and 

 cavities of old craters. 



Beheving the trap-dyke theory to be the most plausible 

 explanation, we would like to ask whether, in spite of no 

 shadows having yet been detected, the rays may not be due 

 after all to Klii/lit riilijis of igneous rock welling out in a 

 viscous state from long cracks, and so catching a little 

 more light than the surrounding parts of the lunar surface. 

 Such ridges might be no more than 100 or 200 feet in 

 height, and if their sides slope gradually it might be im- 

 possible to detect their shadowsi We may also suppose 

 they consist of some light-coloured trap-rock, such as 

 felstone, and to be " weathered " by the lunar atmosphere, 

 thus presenting a somewhat whitened surface. It is quite 

 possible that the lunar trap-rocks maj* be of a highly 

 siliceous nature, like " volcanic glass," also that they may 

 have been considerably weathered and whitened by the 

 action of great quantities of steam, now absorbed by the 

 Moon, emitted in the last phases of lunar volcanic action. 

 We know that steam can act chemically on glass, and turn 

 it white. The lunar photograph in Mr. Ranyard's paper 

 shows that the streaks are not nearly so bright as some 

 moimtains and craters, but this would easily be accounted 

 for by the very great difl'erence in height. Our idea is 

 that the lunar moiintain ranges are composed of volcanic 

 rocks thrown up in some way from lines of fissure, and 

 that the streaks are, as it were, attempts at hmar mountain 

 ranges, which failed because for some reason the lava was 

 not forced up in sufficient quantity. We rebel, for several 

 reasons, against the idea of the lunar mountains being 

 covered with snow. For instance, there is a great differ- 

 ence in the whiteness of different lunar mountains, which 

 would be impossible if snow were the cause of the white- 

 ness. But if they are composed of different kinds of trap- 

 rock, it is extremely likely that they would weather 



differently, so that some might be whiter than others. 

 Those, like basalt, of a more basic character {i.e. with 

 more lime and magnesia), would be of a darker colour, 

 while others, like ielstoue (which is acidic and contains 

 much free silica), would be of a lighter hue. 



In looking over the beautiful pictures in Messrs. 

 Nasmyth and Carpenter's book, we notice another point 

 which seems to favour this idea — namely, that short lines 

 of mountains are so often seen in connection with lunar 

 craters, sometimes roughly radiating from them, some- 

 times all more or less in one direction. We observe this 

 especially in the pictures of Gassendi (the frontispiece), 

 Copernicus, Archimedes, Aristotle and Eudoxus, Trisnecker, 

 Plato, Mercator and Campanus, and also very plainly in 

 the photo of Aristarchus and Herodotus. Again, the 

 occurrence of craters in lines, in some cases, is another 

 important fact tending to confirm this idea. (It will be 

 remembered that terrestrial volcanos run very markedly in 

 lines.) It may be well here to quote the authors above 

 referred to. They say (p. 98) : " We have upon the 

 Moon evidence of volcanic eruptions being the final 

 result of most extensive dislocations of surface, such 

 as could only be produced by some widely diffused up- 

 lifting force. We allude to the frequent occurrence 

 of chains and craters lying in a nearly straight line, and of 

 craters situated at the converging point of visible lines of 

 surface disturbance. Our map wiU exhibit many examples 

 of both cases. An examination of the ujjper portion (the 

 southern hemisphere of the Moon) will reveal abundant 

 instances of the linear arrangement. Three, four, five, or 

 even more crateral circles will be found to lie with their 

 centres upon the same great-circle track ; pro\-ing almost 

 undoubtedly a connection between them, as far as the 

 original disturbing force which produced them is concerned. 

 Again, in the craters Tycho, Copernicus, Kepler and 

 Proclus, we see instances of the situation of a volcanic 

 outburst at an obvious focus of disturbance." 



On this theory, the dark linear markings on the Moon, 

 known as "hills" or "clefts," are probably cracks up 

 which, for some reason, the molten matter only welled-up 

 to some point below the surface. Perhaps they formed 

 later than other terrestrial features, after the volcanic fires 

 had died out, and when the linear surface was losing its 

 old heat rapidly and therefore cracking as it contracted on 

 cooling. 



It must be confessed that there is httle to be said in 

 favour of the view that the lunar streaks have been pro- 

 duced in a similar way to terrestrial " faults," for several 

 reasons : First — the mountains of the Moon, as far as we 

 can see, are different to terrestrial mountains, and seem to 

 be entirely volcanic, whereas our mountains are mostly 

 due to the upheaving and folding of sedimentary strata ; 

 their present outlines being the result of long-continued 

 atmospheric denudation. Secondly — it seems to me im- 

 possible, in the present state of our knowledge, to say 

 whether stratified rocks are present on the lunar surface. 

 If at one time there were seas, and an atmosphere at all 

 like ours, " denudation " must certainly have taken place, 

 and that would involve the accumulation of marine sedi- 

 mentary deposits. Many beheve that there is endence of 

 stratification and even of tilted strata in the hmar 

 Apennines ; but if this is the case, I should prefer to 

 consider such strata as purely volcanic, viz., lava and 

 ashes. Thirdly — terrestrial " faults " are very sharp Unes 

 of dinsion, Uke the cracks which form in a sheet of ice 

 after continued skating, so that we could not expect to see 

 them. 



One word in conclusion about " trap-dykes." These 

 are veins of eruptive rock (basalt, &c.) filling up vertical 



