214 



NATURE 



[May i, 19 1 3 



wearing - constantly a few twigs contained in a small 

 bag or basket slung to the wrist, in the assurance 

 that so good fortune will follow. But few other than 

 chiefs are fortunate enough to possess this valuable 

 jetsam. The twigs are also used as a medium of 

 barter, a fragment of a colony, say, a branch seven 

 or eight inches long, with its associated branchlets, 

 having the exchange value of half a dozen pigs — 

 the staple wealth of the island — or a wife. 



In view of these interesting customs a few particu- 

 lars regarding the Alcyonarian colony referred to are 

 given. The specimen in the museum is a large and 

 much-branched Gorgonid axis, intensely black in 

 colour, with shiny surface marked by many fine 

 grooves and ridges, and entirely devoid of flesh or 

 spicules. By the characters of colour, branching, and 

 general structure of the axis it is clearly a "black 

 coral" — the "schwarze Horncoralle " — Gorgonia (now 

 Plexaura) antipathes of Esper, or some closely related 

 species. Such forms are widely distributed in 

 Oceania, and are known to the natives by various 

 names, frequently signifying "sea-wood," "sea-roots," 

 "iron-wood," and such like. The present specimen is 

 more than 2 ft. high, but examples 5 or 6 ft. high 

 are on record. The colonies are most frequently cast 

 ashore after storms, but they grow in five to six 

 fathoms off Amboina, and belong to a typically 

 shallow-water family. 



It is natural that the large and thick stems of very 

 old colonies used by East Indian peoples for the manu- 

 facture of ornaments should be considered of great 

 value, but it is peculiar that the small twigs of an 

 Alcyonarian widely distributed in Polynesia, an in- 

 habitant of shallow water, and therefore likely to be 

 often cast ashore, should be sufficiently rare in Santo 

 to be a highly coveted medium of exchange. 



James Ritchie. 



The Royal Scottish Museum, Edinburgh, April 18. 



Mechanically-formed Grikes in Sandstone. 



The grikes, or channels, frequently formed by the 

 corrosive action of carbonic and organic acids upon 

 the surfaces of exposed limestone beds are known 

 to most geologists, but a case I recently met with, 

 where similar surface-features have been naturally 

 produced upon a sandstone ledge footing a part of 

 the cliff at Orcombe Point, near Exmouth, is, I ven- 

 ture to think, a phase of marine erosion sufficiently 

 unusual to merit a detailed description. 



Here the Red Marls, with intercalated sandstones, 

 rest on Permian (?) Sandstone. This, owing to its 

 superior hardness, forms a ledge rising abruptly from 

 the beach to a height of about 9 ft. This ledge of 

 sandstone, which has a fairly level surface, is backed 

 by a mass of a somewhat softer variety in the cliff, 

 which, at its greatest distance, is about 20 ft. from 

 the margin of the ledge. 



The surface of this ledge is grooved and channelled 

 to a remarkable extent, and presents an appearance 

 similar to that of limestone grikes. The longest 

 channel has a length of between 15 and 16 feet, the 

 deepest is 2 ft. 3 in. in depth, and the widest has a 

 width of about 2 ft. at the top. In this widest 

 channel is a ridge, about 1 ft. from the top, running 

 along its centre, clearly representing an eroded part- 

 ing which originally separated this widest channel into 

 two parts. All the channels run seawards, and are 

 deepest near the edge of the ledge. Thev are verit- 

 able canons in miniature. A photograph taken from 

 a point above the surface of the ledge is here repro- 

 duced. 



These unusual features have been produced as 

 follows : — During stormy weather the pebbles, grit, 

 NO. 2270, VOL. 91] 



and sand on the beach are cast upon the ledge by 

 the waves. The advancing and receding water causes 

 the beach material to move to and fro on the top of 

 the ledge, and thus the pebbles and sand have literally 

 sawn out these grooves, and the work of deepening 

 and enlarging still goes on. 



The position and direction of the channels was 

 probably originally determined by slight "fossil" 

 ripple-marks which existed on the surface of the 

 rock. 



The sandstone forming the ledge is a fairly hard, 



red, ferruginous rock, composed chiefly of quartz 

 grains, the larger being smooth and rounded, or sub- 

 angular, and the smaller rough and angular. The 

 beach material is chiefly made up of quartz, quartzite, 

 vein-quartz, sandstone, flint, jasper, and shell frag- 

 ments. Cecil Carus-Wilson. 

 April 21. 



Gain of Definition obtained by Moving a Telescope. 



The phenomenon described by Mr. M. E. J. Gheury 

 in Nature of March 27, p. 86, is familiar to me in 

 telescopic work. 



Many years ago, when I used to sweep for comets, 

 sometimes nebulae would be seen to enter the field 

 which were so faint that when the telescope came to 

 rest they were only just discernible or invisible alto- 

 gether. By slowly swinging the telescope back and 

 forth they would become readily visible, as if the 

 process of motion had the effect "of greatly multiply- 

 ing their light. This was not an unusual occurrence. 

 I remember also that it made quite a difference as to 

 whether the object entered from the right or left side 

 of the field. It was easier to detect a very faint 

 nebula or comet when it entered from a certain side. 

 I cannot now remember whether this was from the 

 right or left (the sweeps being horizontal), but I know 



