February 23, 191 1] 



NATURE 



555 



largely. Yet even here (p. 89) certain unknown primitive 

 miners sought tin in fairly deep diggings before the pre- 

 sent natives occupied the coimtry. Ihe same author (vol. 

 .\ii., 1910, p. ibisj reviews all the occurrences of tin-ore 

 in South Africa, and concludes that cassiterite in workable 

 quantities is a product of dirterentiation in granitoid 

 magmas, and that literal secretion accounts for its con- 

 centration in certain veins. 



Mr. A. L. Hall (vol. xii., p. 8) describes schistose struc- 

 tures in the liushveld granite as having arisen marginally 

 through pressure during consolidation. Mr. H. Merensky 

 (p. 13J, in a short but important paper, urges that the 

 diamonds of Luderitziand, in German South-West Africa, 

 which occur in an aH>lian sandstone, must be derived from 

 an underlying sandstone, which he proves to be of Creta- 

 ceous age. Mr. P. A. Wagner (vol. xiii., 19 10, p. 56) shows 

 that dykes of monchiquites, allied to kimberlite, occur in 

 the Pomona district of this region, and in this district the 

 largest diamonds have been found. Prof. R. B. Young, 

 of Johannesburg (vol. xii., p. 82), supports the view that 

 the gold of the banket conglomerate of the Rand was 

 imported, with the pyrite, after the deposition of the beds. 

 He believes that a heavy mineral, such as titanic iron-ore, 

 was present as an original detrital constituent, and pro- 

 moted the precipitation of auriferous pyrite. He traces a 

 second generation of gold, distributed more irregularly 

 than the first. He suggests that the gold was brought in 

 by solutions arising from igneous rocks, both basic and 

 acid, that penetrate the W'itwatersrand series. The acid 

 intrusive rocks have been described for the first time by 

 Mr. M. Weber C»6td., p. 67), who has detected gold in 

 them. The future must show whether the gold in these 

 igneous rocks has or has not been derived from other 

 rocks through which they have passed in their ascent. 



A question that attracts even more interest in South 

 Africa is raised by Mr. H. S. Harger's paper (p. 139) on 

 the occurrence of diamonds in Dwyka conglomerate and 

 amygdaloidal lavas, and the origin of the Vaal River 

 diamonds. Mr. Harger is a specialist in diamond-bearing 

 pipes, and he believes that some of the material in old 

 alluvial gravels above the Vaal River has been derived 

 from local kimberlite. He regards, however, most of the 

 blocks associated with the diamonds as torn from more 

 distant masses by the ice of Permo-Carboniferous times. 

 The gravels are, in fact, concentrates from lost patches of 

 Dwyka conglomerate. He shows that the so-called 

 " bantam " pebbles, commonly associated with diamond 

 on account of their specific gravity of 3-3, are probably 

 worn from a metamorphic rock rich in manganese-garnet, 

 and he traces these pebbles to the Dwyka beds. He 

 finds, moreover, diamonds in the andesitic lavas that are 

 older than these strata, and urges that the gems originated 

 in these lavas, and were carried thence into the con- 

 glomerate, and thence into the residual gravels. In the 

 discussion reported on pp. Ivii-lix of the Proceedings of 

 the society for 1909-10, Mr. Harger defends his position 

 by recording the occurrence of diamond in the Dwkya 

 conglomerate at Windsorton. He does not, however, 

 encourage the exploitation of this intractable and uncon- 

 centrated series. We do not seem nearer to the actual 

 parent rocks of the diamond, which may well lie in some 

 metamorphic zone, from which the gems became picked 

 off into the lavas. Mr. C. Baring Horwood (vol. xiii., 

 p. 29) publishes and discusses a number of analyses of 

 typical Transvaal rocks, including the dolomite and its 

 partly silicified varieties. In association with Mr. A. 

 Wade, he has recently reviewed the whole series of " old 

 granites " in South Africa (Geological Magazine, 1909, 

 PP- 455 ^nd 497), and concludes that there is a real funda- 

 mental granite-gneiss formation in that portion of the 

 globe. The state of affairs, however, as he fairly enough 

 points out, is somewhat suspiciously like that in Canada, 

 where the fundamental series tends to become more and 

 more visionary every year. Dr. Rogers, in his address to 

 the South African Association for the Advancement of 

 Science, in November, 19 10, clearly differs from Mr. Hor- 

 wood in regard to the African series, and points out that 

 the oldest gneisses are igneous intrusions including flakes of 

 sediments (Reports, Section B, p. 30). 



Mr. F. P. Mennell (Quart. Journ. Geo!. Soc. London, 

 vol. Ixvi., 1910, p. 353) claims the great mass of rocks in 

 southern Rhodesia as " Laurentian "; but he is convinced 

 NO. 2156, VOL. 85] 



that the granitoid mass which forms so large a part of the 

 country is younger than the series of schists, banded iron- 

 stones, and limestones, and he holds that mixed rocks are 

 important features of the contact-zones. The present 

 writer has had the advantage of seeing some of these 

 composite gneisses under Mr. Mennell's guidance near 

 Bulawayo. Interesting cases of the absorption of granite 

 by dolerite, recalling the reverse action near Carlingford in 

 Ireland, are described on p. 372. Mr. Mennell, in referring 

 to two Rhodesian examples of " blue ground " pipes con- 

 taining diamonds, declines to connect the diamonds with 

 the prevalence or non-prevalence of eclogite-fragments or 

 of garnet. This is in contradiction to the view of the 

 Vaal River diggers, as quoted by Mr. Harger in the 

 paper already referred to, since the " bantams " on which 

 they so much rely prove to be largely made of spessartine. 

 Geologists may well envy the field open to Mr. Mennell, 

 Mr. Molyneux, Mr. Zealley, and now to Mr. Maufe, who 

 between them are attacking an area at least as large as 

 the Transvaal. G. A. J. C. 



THE AIRSHIP FOR THE BRITISH NAVY. 

 ''PHE leading article in Engineering for February 17 gives 

 •^ some account of the airship for the British Navy 

 built by the \'ickers Company at Barrow. Trials were 

 conducted on Tuesday, February 14, in presence of the 

 Government's Advisor}- Committee on Aeronautics, these 

 being analogous to the basin trials of a warship, and 

 have proved to be quite satisfactory. The structure for 

 accommodating the hydr<^en reservoirs or balloons is 

 512 feet in length and 48 feet in diameter. It is in the 

 form of a decagon in section, and the ten sides are built 

 up of longitudinal lattice-girders, with vertical intercostal 

 girders, the top and bottom boom in each case being 

 formed of angles or tees of duralumin. Each bay has 

 diagonal wire bracing. The form is whale-like, with a 

 bluft entry, and a sweet run aft to a point, where, at the 

 bottom, there is a big fin, increasing in depth aft accord- 

 ing to the upward rise to the point of the stern. 

 Aluminium was first tried, but the girder structure of this 

 metal collapsed under stress. The metal adopted — 

 duralumin — is one of the magnesium alloys of aluminium, 

 and contains 91 per cent, of aluminium. It has a specific 

 gravity between 2-77 and 284, a melting point of about 

 650° C., a yield point varying from 12 to 16 tons per 

 square inch according to the hardness, and a breaking 

 resistance from 22 to 29 tons per square inch. The 

 elongation varies from 23 to 18 per cent., and the con- 

 traction of area from 34 to 26 per cent. It will thus be 

 seen that, despite its lightness, it bears comparison with 

 mild steel. 



For more than half the length of the structure there is 

 a bottom girder, or keel, of V shape, carried on the 

 girder structure of the decagon. The bottom is flattened 

 with spruce grating, laid inside to form a gangway, and 

 serves as a means of communication between the two 

 gondolas. The gondolas are connected to the central 

 girder, and are constructed of timber of ship-shape form. 

 Should the ship alight on water, the structure will float 

 by reason of the buoyancy afforded by the hydrogen gas 

 contents of the reservoirs. Both gondolas contain a typical 

 ten-cylinder Wolseley marine petrol-motor with reversing 

 clutch. The engine in the forward gondola has two pro- 

 pellers, each with two wooden blades. There is one on 

 each side at a considerable elevation above the gondola, 

 supported on duralumin raking girders. The engine in the 

 after gondola drives a single two-bladed propeller abaft 

 the gondola, with only a reversing coupling between pro- 

 peller and engine. 



To give lifting power, eighteen or twenty gas-bags are 

 used, the structure for the hydrogen reservoirs being 

 divided vertically into compartments by rope netting. The 

 covering of the structure was the subject of experiments 

 at the National Physical Laboratory, and, as a result, silk 

 coated with a proofing by the loco process was preferred. 

 This weighs about 100 grams per square metre, has fire- 

 resisting qualities, and is of British manufacture. The 

 upper half is coated with aluminium dust in order to 

 reflect the sun's rays, while the lower half retains the 

 yellow shade of the silk. 



