Janl-arv. 1911. 



KNOWLEDGE. 



27 



Hoiiialoiiiyia canicitlaris. is to be studied. Dr. Graliaiu- 

 Sinitli will continue his work on the e.xperimental infection of 

 flies with pathogenic micro-organisms, whilst Dr. Nicoll will 

 inquire into the possible agency of flies as carriers of the eggs 

 of parasitic worms, and so on. 



These are a few of the lines of research and experimentation, 

 all of which will increase our knowledge and place us in a 

 more able position to effectively deal with a pest we can well 

 aftbrd to be without, and one in dealing with which the general 

 public can materially assist. 



GEOLOGY. 



By RussKLL F. GwiNNELi., B.Sc. .A..K.C.S.. F.G.S. 



WIND .-WD WAT1:R. — Quite a number of papers 

 published within the last few months deal with such subjects 

 as the movements of wind and water, and the resulting aeolian 

 and aqueous deposits. 



In "The Origin and Growth of Ripple Marks" iProc. Roy. 

 Soc. A.,Vol. l.xxxivl. Mrs. .Ayrton describes experiments leading 

 to the conclusion that there are three ways of originating sand 

 ripples under water. Two of these methods are capable of 

 originating and then extending regular ripple- marks wherever 

 water is oscillating over sand, whether its surface be smooth 

 or uneven to start with. These are : — 



(1) The "Differential Motion" Method, by which single 

 ridges arise at the loops of stationary wa\es. 



(21 The "Brush" method, whereby the brushing action of 

 the vortex in the lee of any existing dune or obstacle 

 sweeps up a new ridge beside it, leaving a hollow 

 along the neutral line of the vortex. 



The third method, depending on an initial uneven surface, 

 could, unaided, give rise to irregular ripple-mark only, since 

 it consists in piling up sand in places where the surface 

 chances to be uneven. 



Summarily the processes by which mounds of sand are 

 built up under the action of stationary waves are : — 



( 1 ) Ripples alwa\'s first form at places of maximum longi- 



tudinal motion of the water, i.e., at places of constant 

 level. 



(2) Smooth spaces are left at and near the places where the 



water has no longitudinal motion, i.e., where the 



change of level is greatest. 

 (31 liach set of ripples grows into a mound having its centre 



plane at the place of maximum longitudinal velocity 



and its lowest parts close to places where the 



longitudinal motion is zero. 

 (41 There are therefore two of these mounds to each water 



\va\e. 



Mr. -A. Wade, B.Sc, F.G.S., in the Geological Magazine 

 September, 1910, deals with "The Formation of Dreikante 

 in Desert Regions." These curious three-edged stones, so 

 characteristic and abundant in most desert regions, are shaped 

 somewhat like a Brazil nut. The old explanation of their 

 fluviatile origin has long been discarded in favour of an aeolian 

 origin, which supposes that the wind I laden with sand and 

 thus a powerful eroding agent) strikes the point of the pebble, 

 glancing off on either side so that the " keel " on the upper 

 surface of the pebble is in the direction of the wind. If this 

 is the true explanation, it is difficult to make it tally with 

 observations recently carried out by the author in the 

 Egyptian Desert. 



Thus he finds that the Dreikante are equally pointed at both 

 ends; collecting over three hundred specimens in an area of 

 about three hundred acres, he finds 78% set with the " keel " 

 appro.ximately at right angles to the wind; the keel instead of 

 being a simple straight one is often curved or even bifurcating 

 at both ends, giving a five-faced solid instead of a three-faced 

 one ; the faces are parabolic cur\es ; finally, actual experiment 

 proved that these three-edged stones ccjuld not remain with 

 their pointed ends facing a strong current of wind. 



The author, therefore, shows that the travelling sand-grains 

 hit not edgewise but broadside on a face ; and being projected 

 off again in an upward direction, the parabolic cur\e is 



produced. The cutting-away continues until the pebble 

 stands on a very narrow base, and then topples over ; thus the 

 erosion commences again on a fresh surface. " Sometimes at 

 night the rattling of the pebbles moving under the action of a 

 high wind is very considerable." 



In the November issue of the same magazius is printed a 

 summary of the views of the Russian geologist, Iv. ':,enko, as to 

 aeolian deposits. He classifies the various types of stratifica- 

 tion in aeolian deposits (such as ripple-mark, diagonal 

 stratification w-ith opposed dips, horizontal, vertical, and so on), 

 and their modes of origin. In addition to ordinary winds the 

 action of whirlwinds can often be distinguished by the spiral 

 or circular arrangement of the dust particles. The dimensions 

 of particles of desert dust transported by the wind diminish 

 from the centre towards the margins of the desert. The 

 grains of sand have special characters in aeolian and in 

 aquatic deposits ; in the former they are characteristically 

 triangular. 



Finally, we may note two papers by Mr. A. R. Horwood, of 

 Leicester Museum, on " The Origin of the British Trias." 

 One paper was read before the British Association in 

 September last, the other before the Geological Society on 

 November 23rd. The general view of conditions in Triassic 

 times is that the deposits appear, on the whole, to have been 

 laid down under conditions similar to those which now prevail 

 in the dry interior of continents. The lower-lying country 

 between the mountain ranges was covered by wind-blown 

 sand and alluvial deposits washed down from the hills during 

 occasional floods, and gradually the mountain-chains were 

 more or less completely buried. In hollows in the nearly level 

 plains that were thus produced, the water collected and was 

 evaporated, forming lakes of brine. (See Lake and Rastall : 

 "Text Book of Geology," 1910). 



Mr. Horwood, however, considers the British Trias to be a 

 delta deposit, in which desert conditions did nothing more 

 than locally act upon the rock. They had no part whatever 

 in the icorl; of deposition. Among many other points, he 

 notes that ripple-mark affects only the sandstones, and not the 

 marls ; also that, whereas all varieties of desert sand agree in 

 possessing the texture of miniature pebbles with a highly 

 polished surface, all Trias sediments exhibit an eroded surface. 



He maintains that there is positive, direct and accumuLative 

 evidence to prove that the Trias as a whole (and not the 

 Bunter only) was the work of rivers which had continued to 

 bring sediment in one form or another from the north-west of 

 Britain more or less continuously from the close of the marine 

 phase of Lower Carboniferous times. 



THF ORIGIN OF CORAL ISLANDS.— Reef-building 

 corals can only thrive in seawater which does not exceed 

 about thirty fathoms in depth, and the temperature of which 

 never falls below 20" C. (68° F). The latter limitation almost 

 restricts coral islands to tropical seas ; the former implies a 

 shallow submarine foundation in every case. The explan- 

 ation of the occurrence of such shallow foundations or 

 submarine plateaux, far from continental land, has been more 

 or less of a mystery right down to the present day. The fact 

 of the existence of the plateaux is established, and also that 

 the material is frequently volcanic ; in other cases the living 

 coral is founded on an enormous thickness of dead coral rock 

 (as at Funafuti, where a boring of over a thousand feet 

 encountered nothing but coral limestone). 



The curious atoll-structure, where a ring of coral islets 

 more or less encloses a central lagoon, renders the explanation 

 more difficult. The main theories are those of Darwin, 

 Dana, and others, on the one hand, and of Murray, Agassiz, 

 on the other. Darwin's hypothesis necessitates subsidence 

 of innumerable islands, the corals growing upwards on a 

 talus-slope of their own debris as fast as the islands sink. 

 This implies widespread crustal subsidence. In Murray's 

 eyes the submarine platforms are volcanic islands denuded to 

 a " plain of marine erosion." and the conditions are those of 

 general elevation or long-continued crustal repose. 



R. A. Daly, writing in the American Journal of Science 

 (November, 1910) on Pleistocene Glaciation and the Coral 



