282 



KNOWLEDGE. 



August, 1913. 



that they are underground rivers. The Oueds of the 

 Sahara are buried rivers, but to-day no rivers are to 

 be seen, only more or less fertile valleys in the desert 

 marking their courses. The rivers have been buried 

 by the drifting sand, but their waters still pass 

 along the old channels. In a different way we 

 suggest that the waters of the Downs have become 

 underground streams, which have made their own 

 now streamless vales. Suppose the line of such a 

 valley lies among the main dip, and that rolls in the 

 chalk cause minor dips on both sides towards this. 

 The rain sinking into the chalk will descend till it 

 reaches an impervious substratum. Then it will 

 soak on both sides towards the central lines of the 

 valley. Reaching this it will, along with what 

 reaches it directly from the surface, flow or soak 

 along the main dip. All the time it is dissolving 

 and carrying away the rock, and the surface 

 gradually sinks in, as it does in the formation of 

 swallow holes. Thus we suggest that the dry 

 valleys of the chalk are due to subsidence caused by 

 underground waters directed by the slopes of the 

 strata. 



The action of the underground water would 

 be assisted in a small degree by that of the sur- 

 face. For when a channel was once formed the 

 rain water would tend to collect in the hollow in 

 the form of soaked soil or subsoil. Here, then, the 

 solvent action would be greater, and the making of 

 the valley hastened. The whole is a tentative 

 hypothesis only, but one that seems worthy of 

 careful consideration. And, apart from any inherent 

 probabilities in their respective suppositions, it 

 appears to possess this advantage over the glacial 

 hypothesis, that it views the valleys as still in the 

 making. Mr. Reid's explanation, on the other hand, 

 looks upon them as completed in the Glacial Epoch. 

 And we think that there is evidence that the denud- 

 ing forces are still at work, for otherwise would 

 they not show signs of filling up ? There must be 

 a certain amount of slipping and sliding and wash- 

 ing of matter into the valleys from the sloping sides 



which would tend to obliterate them. But they do 

 not, as we have said, appear to be growing less. 



It is possible, however, that some of these valleys 

 date back for their initiation to the time when the 

 chalk was covered by Tertiary deposits, sandstones, 

 clays, limestones, and so on. In these strata 

 streams might cut out valleys in the ordinary way. 

 When these rivers had got down to the chalk, their 

 beds would contain sufficient clay, and so on, to 

 prevent them altogether being absorbed. The chalk 

 itself might have become impregnated with clay to 

 a certain depth, and so have become impervious. 

 This might last long enough for the stream to cut 

 down the chalk to a certain extent. Finally, how- 

 ever, all clay and impervious chalk would be swept 

 away, and the rain sink directly into the ground as 

 it does to-day. But we need not suppose that the 

 river which cut the valley has actually gone. It 

 may, so to speak, have sunk into the ground, and be 

 still carrying on its work of excavation. Obviously, 

 at any rate, the water which normally runs off as 

 streams and rivers is in some fashion working its 

 way through the rocks below. 



We may carry the idea of underground streams 

 and rivers a little further. The invisible waters of 

 the downs flow finally into a great subterranean 

 lake which lies in the rocks beneath the lower 

 valley of the Thames. There they are joined by a 

 similar set of streams from the chalk hills north of 

 London. For the chalk of the south of England 

 dips beneath the strata of the London basin, and 

 rises again to the north. The Tertiary rocks of the 

 London basin, in fact, lie in a syncline or trough of 

 the chalk, and in this the water collects. So a part 

 of the rain falling on the Downs finds its devious 

 way to the great subterranean reservoir over which 

 is built the Metropolis. When an artesian well is 

 sunk through the Tertiary beds and a part of the 

 chalk an abundant water supply gushes out. The 

 waters which should " delight our broad and brook- 

 less vales " go unseen to supply the deeper wells of 

 the city of London. 



ADDITIONS TO THE ZOOLOGICAL SOCIETY'S MENAGERIE. 



The registered additions to the Society's Menagerie during 

 the month of June were 295 in number. Of these 144 were 

 acquired by presentation, 35 by purchase, 56 were received on 

 deposit, 15 in exchange, and 45 were born in the Gardens. 

 The following may be specially mentioned : — 



One Pudu Deer (Pudupudu), from Chili, and two Patagonian 

 Cavies (Dolichotis magellanicus), from Patagonia, presented 

 by Mr. Albert Pam, F.Z.S., on June 2nd. 



Three Pumas (Felis concolor), born in the Menagerie on 

 June 13th. 



Two Canadian Beavers (Castor canadensis), from Canada, 

 received in exchange on June 5th. 



One Savanna Sparrow (Passerculus savannah), from 

 North-East America, new to the Collection, purchased on 

 June 18th. 



One Ceylon Mynah (Acridotheres melanosternus), new 

 to the Collection, presented by Dr. Philip H. Bahr, F.Z.S., on 

 June 29th. 



One Golden-fronted Woodpecker (Melanerpes flavifrons), 

 from Brazil, new to the Collection, purchased on June 5th. 



One Calthrope's Parrakeet (Palaeornis calthropae) from 

 Ceylon, new to the Collection, deposited on June 29th. 



One Condor (Sarcorhamphus gryphus), from Chili, pre- 

 sented by Mr. Albert Pam, F.Z.S., on June 2nd. 



Two Crested Screamers [Chauna cristata), bred in the 

 Menagerie on June 19th. 



Two Sun-Bitterns (Eurypyga helias), from South America, 

 purchased on June 16th. 



One Kagu (Rhinochetus jubatus), from New Caledonia, 

 received in exchange on June 27th. 



Two Spiny -tailed Skinks (Egernia depressa), from 

 Australia, new to the Collection, purchased on June 10th. 



A collection of Snakes from Sierra Leone, including three 

 Sooty Snakes (Boodon fuliginosus), new to the Collection, 

 presented by Mr. Guy Ayhner, F.Z.S., on June 18th. 



A collection of Snakes from India, including one Forsten's 

 Tree-Snake (Dipsas forstenii), new to the Collection, received 

 in Exchange on June 2nd. 



Two Gopher Frogs (Rana aesopus), from North America, 

 new to the Collection, received in exchange on June 2nd. 



