]30 



KNOWLEDGE 



[JlLY 1, ItidS. 



volcanoes, and other markings evidently due to differences 

 of albedo, remain visible when the sun is at all altitudes. 



One of the most remarkable of these veiled crater forms 

 is traceable in the photograph on the Mare Tranquillitatis, 

 a little to the west of the craters Arago (63) and 

 Manners (486). In the photograph of the Messrs. Henry 

 it is distinctly seen to be a double crater, surrounded by 

 streaks or ridges which radiate from it. One of these, a 

 double ridge, is well shown in our plate, trending up 

 towards the circular crater Sabine (65). Though these 

 ridges are not very lofty, they extend over an area which is 

 very extensive compared with any of the volcanic areas 

 on the earth. Thus the circular crater Theophilus (319) 

 is stated by the Eev. T. W. Webb to be 61 miles in 

 diameter — that is, it about corresponds in area to the 

 county of Devon. The shallow or submerged crater on 

 the Aiare Tranquillitatis is nearly as large as Theophilus, 

 and the ridges or lava streams which radiate from it 

 extend over an area as large as Ireland. 



In many instances we are able to determine the order 

 in which lunar formations have made their appearance. 

 Thus, Fracastorius (372), the great crater to the south of 

 the Mare Ncctaris, was evidently formed before the Mare 

 Nectaris, or, at all events, before an overHow of matter 

 from the Mare Nectaris broke into and partially obliterated 

 the northern part of the ring about Fracastorius. It 

 -will be noticed, however, that the northern arc of the 

 Fracastorius ring is still traceable, though it has evidently 

 been much reduced in height by the matter which has 

 overflowed from the Mare Nectaris. This is by no means 

 a solitary instance ; there are many other such partially 

 obliterated rings upon the moon, indeed there are several 

 traceable upon our plate — see the two half rings to the west 

 of Maskelyne (57), and the incomplete large crater in the 

 northern part of the Palus Somnii (F). The crater .Jansen 

 (66) seems also to have been partly broken in upon on its 

 south-eastern side, that half of the ring being decidedly 

 less lofty than the north-western half. There is also a 

 partly obliterated large ring or polygonal area around 

 Torricelli (318) in the Mare Frigoris, which seems to have 

 been filled up with liquid matter, for the area within it is 

 much smoother than the surrounding parts of the ^lare 

 Frigoris. The northern part of the outline of this area is 

 more obliterated than the southern, as if the matter which 

 filled up the crater had flowed into it from the direction of 

 the Mare Tranquillitatis. On the other hand, the polygonal 

 area may correspond to an overflow of matter from 

 Torricelli (318) or to a crater which has been filled up by 

 matter welling up from its own central orifice ; but, if so, 

 the polygonal crater must have been a mere depression 

 without a surrounding wall. These partly obliterated 

 crater forms, as well as the broken ring about Fracastorius 

 (372), seem to point to the partial levelling of mountain 

 forms by the action of a liquid which did not sweep away 

 the ridges altogether and silt up the interiors of the craters, 

 leaving no vestige of the previous formations, as might be 

 expected if the obliterating action were due to water and 

 the sediment deposited by water, but the submergence of 

 the lunar formations seems to have been only partial, 

 lea\ing relics of the previous formations which are clearly 

 traceable on the new surface. 



The theory that the lunar plains were formed Ijy an 

 outpouring of lava over an enormous area of the moon's 

 surface would refer back their origin to a very early period 

 in the moon's history, and the date of the formation of 

 the craters invaded or partially obhterated by the 

 lunar plains would be carried back to a still earlier 

 period. It will be noted that these earlier craters do not 

 jjresent a distinctively roimded appearance indicating a 



longer period of weathering, or any obvious characteristics 

 which dift'erentiate them in a striking manner from craters 

 formed upon these lunar plains, of which there are many 

 visible in our plate, ranging in size from the most 

 minute cup-shaped depressions up to craters of the size 

 of Plinius (61) and Arago (63). 



There are also to be seen, standmg upon the 

 lunar plains, many walls and detached masses which 

 one would not at first sight recognize as having a 

 volcanic origin, unless they correspond on a gigantic scale 

 to the fumaroles which are occasionally built up on terres- 

 trial lava lakes. One of the most curious of these forms 

 will be recognized on the photograph to the south of 

 •Jansen (66). It seems to consist of two walls which meet 

 at an acute angle. It is very white, compared with the 

 general tint of the Mare Tranquillitatis surrounding it, 

 and from near the southernmost end of the longer branch 

 a series of isolated masses are arranged in a line which 

 trends oft' towards the south-east. There is another curious 

 white wall standing out upon the plain between the Alare 

 Tranquillitatis and the Mare Frigoris. It is a little to the 

 west of the promontory which lies to the north-west of 

 Hypatia (317), and a little further west of this is a curious 

 bent wall, the northern and southern parts of which meet 

 at an obtuse angle. They may possibly have formed the 

 western boundary of a polygonal submerged crater some- 

 what similar to that around Torricelli, for there are traces 

 of a southern wall joining up the southern end of the bent 

 wall with a small craterlet on the edge of the rising ground 

 to the west of Hypatia. 



Stretching from the southern end of the white promon- 

 tory to the south-west of Hypatia is a curious narrow 

 dark line, which seems not to be due to any photographic 

 defect. It skirts the high ground on the edge of the mare 

 for a distance of more than a hundred miles, and ultimately 

 seems to pass through or over a ridge to the south of 

 Sabine (65). In connection with this dark line, I would 

 call attention to a comparatively narrow dark marking 

 which skirts the western edge of the Mare Nectaris. It is 

 somewhat broader than the dark line on the edge of the 

 Mare Tranquillitatis, but it seems to skirt the high 

 ground in a somewhat similar manner. 



Nott cr of B oolt. 



An Account of British Flics [Diptera]. Vol. I. By F. V. 

 Theobald, B.A., F.E.S. (Elliot Stock).— Of this new ven- 

 ture, the introductory part of which we noticed at the 

 time of its publication, the author has now completed his 

 first volume. Thus far, six families have been dealt with, 

 comprising chiefly the fleas and most of the so-called 

 gnats. Mr. Theobald's subject in the present volume is 

 in many respects a tempting one. The chief drawbacks 

 the student has to encounter are the extreme fragility of 

 the perfect insects, and the obscurity of theii' colours and 

 forms. In fact, they do not look interesting, and this 

 circumstance, coupled with the dift^iculty of their preserva- 

 tion, has no doubt acted largely as a bar to their systematic 

 study in this country, so that probably at the present 

 moment there are very few people to whom the group is 

 other than a neglected one. And yet it has very much to 

 i-eeommend it. It would be difficult to find in any other 

 order of insects six consecutive families which exhibit so 

 great a variety of habits, or such remarkable diversities of 

 life-history. Some of the species, moreover, are asso- 

 ciated with extremely interesting biological problems, and 

 amongst them are to be found several which have an 

 immense influence on man and his works and ways. It 



