640 



NA TURE 



[October 2X. 1901 



the spoil bulks on either side ; conveyors forming a bridge stretch- 

 ing acrois the channel, with cantilever arms projecting over the 

 spoil lunks on each side, carrying a steel travelling belt which con- 

 veys the material to the depositing ground ; cantilever conveyors 

 running on rails along one bank of the trench, with one arm 

 dipping down into the excavations and the other rising over the 

 spoil bank, up which incline a trolley is drawn for disposing of 

 the earthwork ; inclined planes leading to a travelling bridge with 

 an open roadway extending over the spoil bank, through which 

 wagons drawn up the incline deposit their load ; and, lastly, 

 high-power revolving derricks and other machinery for the 

 rapid and economical removal and deposit of the excavations. 



The novelty of the reservoir dam in progress at Assuan across 

 the Nile consists in the one hundred and eighty sluices by which 

 it is pierced aflfording a waterway of 24,000 square feet, through 

 which the whole flow of the Nile in flood-time will be dis- 

 charged, amounting to a maximum of 475,000 cubic feet per 

 second with a velocity of 20 feet per second. These openings 

 will be closed for storing up water for summer irrigation, by 

 counterbalanced sluice-gates working on free rollers, which can 

 be readily raised or lowered against a considerable head of 

 water. 



The deepening of the navigable channel by about 3i feet outside 

 the Sulina mouth of the Danube since 1S95 by dredging, giving 

 an available depth of 24 feet, shows that it is possible under 

 favourable conditions to cope with the deposits of a minor 

 channel of a deltaic river by means of dredging, at any rate for a 

 time ; though it must lie anticipated that eventually the accumu- 

 lations of deposit -in front of the mouth will necessitate an 

 extension of the jetties, to enable an improved scour across the 

 advancing delta to aid dredging in the maintenance of the depth 

 ot the outlet channel. 



The injuries caused during two successive winters to the super- 

 structure on the top of a rubble mound, forming the main break- 

 water in progress for sheltering the approach to the River 

 Nervion leading to the port of Bilbao, exposed as this breakwater 

 is to the full force of the waves rolling in from the Bay of Biscay 

 during north-westerly gales, has led to the adoption of a novel 

 method of depositing blocks of concrete of unusual size for the 

 purpose of providing a secure foundation for the superstructure 

 in this exposed site, where the breakwater extends into a depth 

 of about 50 feet at low tide. The method comprises the con- 

 struction of metal caissons to serve as a lining for the blocks, 

 which are ballasted with concrete, floated out into position, and 

 sunk in place Ijy filling them with water, after which they are 

 filled as rapidly as possible with large concrete blocks, and with 

 concrete in mass in the interstices and on the top, so as to con- 

 stitute a solid block, the largest blocks thus formed at the 

 Bilbao Harbour Works having a weight of about 1500 tons. 

 These blocks, laid in a row on the top of a rubble mound at a 

 depth of about 16.^ feet below the lowest low water, within the 

 shelter of the original rubble mound with its capping of large 

 concrete blocks, have proved a perfectly stable foundation for 

 the superstructure which is being erected upon them. This 

 system is being extended at Zeebrugge Harbour in the North Sea, 

 at the entrance to the Bruges Ship Canal, where steel caissons 

 have been constructed and lined with concrete, which are to be 

 floated into position in calm weather one by one for the founda- 

 tions of sea and harbour walls along each side of a quay, and an 

 outer solid breakwater ; and these blocks, when completed, will 

 rest on the sea bottom, and weighing from 2500 tons up to 

 4400 tons, will emerge about 24 feet out of water at low water 

 of spring tides, so that a solid superstructure can be readily 

 built upon them. 



Remarkable progress has been achieved in recent years in the 

 extension of appliances for the more efficient lighting of minor 

 shoals, outlying reefs, and navigable channels. The ease of 

 rotation obtained by floating the illuminating apparatus on an 

 annular mercury bath, has enabled the system of group flashes, 

 giving a distinctive character to each light, to be extended to 

 beacons exhibiting a continuously burning light for three or four 

 months, by rotating the light apparatus by an electric battery 

 placed in a chamber in the beacon. The increased speed of 

 rotation, moreover, rendered possible by the floating on a mer- 

 cury bath, has enabled the number of panels of lenses to be 

 reduced and their size increased, and consequently a brighter 

 flash to be exhibited. Various improvements also have been 

 effected in the lights themselves. Thus carbonised wicks have 

 been devised which enable a light to continue burning without 

 being attended to for a considerable period, with only a 



moderate deterioration in intensity ; incandescent lamps have 

 been adopted, fed by oil gas or petroleum vapour, which pro- 

 vide an excellent light ; and acetylene is being experimented 

 upon by the French Lighthouse Servic'e, and the danger of ex- 

 plosion having been overcome by using very small tubes for 

 supplying the burner, it appears likely to furnish a very bright, 

 serviceable light. Special attention has been lately devoted to 

 reducing the divergence of the light exhibited by lightships from 

 the vertical, as with a considerable rolling of the vessel in a storm 

 the light is liable to be obscured for a time. As it has been 

 ascertained by observation that the waves in severe storms have 

 a fairly definite period of oscillation in any particular locality, 

 the special period of oscillation of the waves where a lightship is 

 to be placed is ascertained ; and the vessel is so designed, and 

 its weights adjusted, that its period of roll may differ materially 

 from the oscillation of the waves at its station ; and the roll of 

 the lightship is further checked by giving it a large draught and 

 deep bilge keels. Moreover, the light and its accessories are 

 supported on a sort of compound pendulum, with weights so 

 adjusted at the bottom and above the light that the oscillation 

 of the pendulum difi'ers from the roll of the vessel, and the 

 stability and consequent visiliility of the light is thereby 

 increased. 



Altogether the papers furnish interesting indications of some 

 of the advances being achieved in the execution of waterways, 

 maritime works, and the lighting of shoals and channels ; and 

 the prospect of important extensions of waterways is manifested 

 by the Dortmund and Ems Canal, forming merely the first instal- 

 ment of a waterway intended to connect most of the rivers of 

 Prussia, and the proposal of a Russian engineer for constructing 

 a deep waterway to connect the White Sea and the Baltic, 

 capable of being traversed by large seagoing vessels. 



NO. 1669, VOL. 64 j 



ITALIAN GEOLOGY^ 



A N elaborate memoir, containing results of a study of the 

 "^ rocks and geology of the basin of the Sesia with the ex- 

 ception of its lower portion, the Strona valley and the western 

 portion of the Orta lake, has lately been issued. The authors 

 remark that, having made traverses of this region in several 

 directions, noting many stratigraphical details, they were obliged 

 to recognise the impossibility of the task of determining the 

 "absolute chronological value" of the difl'erent formations. 

 Neither does their microscopic examination of the rocks help them 

 more to unravel the stratigraphical problems. This is a result 

 which is not infrequent where petrographical methods are 

 treated as paramount. Petrography, as I have frequently laid 

 stress upon, is but ait aid to geology, a valuable one, I admit, 

 but inferior to good and accurate field-work, lithology, and a 

 wide general knowledge of the surrounding region, and especially 

 of the habits in other regions of the same class of rocks. 



The authors have, as they but too justly point out, to contend 

 with the absence of any known fossiliferous horizon, or in fact 

 any stratigraphical standard formation as a datum to work from. 

 In addition a large mass of volcanics traverse the Valsesia 

 between the two principal crystalline formations and produce 

 uncertainty in the limits of each, further disturbing the already 

 complex stratigraphical arrangement and masking the relations 

 of one to the other. At the commencement of the paper is 

 a bibliographical list of fifty-three memoirs dealing with the 

 locality in question. 



It was lound convenient for the petrographical studies to 

 divide the rocks of the higher basin of the Sesia into five 

 groups: — 



(i) Gneiss of Strona (with an appendix on the granites). 



(2) Massive augitic and hornblendic rocks. 



(3) Gneiss of Sesia (including the schists of Rimella and 

 Fobello). 



(4) Greenstones (pietre verde) properly so called. 



(5) Gneiss of Monte Rosa. 



The authors deserve much credit for not venturing beyond the 

 old nomenclature of Gerlach and Parona, the earlier students of 

 this region. 



Under the first group are included mica-schists with silli- 



1 "Ricerclie Petrograliche e Geologiclie sulla Valsesia," by E. Arlini 

 and G. Meizi (;l/,v«. <i,:t R. Istitnto Lomlardo di Sc. e Lett., vol. xvii. 

 pp. 219-392 ; pi. x.\ii.). 



