May, 1913. 



KNOWLEDGE. 



183 



GEOLOGY. 



By G. VV. Tyrrell, A.R.C.Sc, F.G.S. 



LONDON WELLS. — The water-supply of London is of 

 such tremendous importance that contributions to its fuller 

 study are extremely desirable. Hence the Geological Survey 

 has accomplished a valuable piece of work in compiling an 

 exhaustive record of London wells. The information as to 

 this mode of water-supply is scattered, and in the older 

 records incomplete. So far as is possible this incompleteness 

 has been remedied, and a very large number of borings made 

 during the last few years have been included in the Memoir, 

 which has been compiled by G. Barrow and L. J. Wills. The 

 introduction by Mr. Barrow is a valuable account of London 

 water-supply, and describes the strata penetrated by the wells, 

 methods of well-sinking, relation of water-supply to geological 

 structure, and many other kindred subjects of interest. Part 

 II consists of a catalogue of published London wells, and 

 Part III of a descriptive list of new London wells with some 

 old ones. 



Special attention is drawn in the introduction to the 

 continued fall in the water-level under London. The fall is 

 greater than was anticipated, and has recently been taking 

 place at an increased rate. The fall is illustrated by contour- 

 maps of the water-level at different dates in the London area. 

 These show very clearly the setting up of specially low water- 

 levels, and the gradual outward spread of the low contours 

 from these areas. The fall may be illustrated by the three 

 wells sunk in 1847 to supply the fountains at Trafalgar Square. 

 These once produced five hundred and eighty thousand 

 gallons a day, and supplied several government offices beside 

 the fountains. Only one is now in use, and supplies only 

 eight thousand gallons an hour, and in this well the water-level 

 has fallen one hundred and fifteen feet in sixty-four years, 

 A similar instance is offered by the deep well sunk in 1864 to 

 supply the fountains at the head of the Serpentine in Kensington 

 Gardens. There is a general coincidence of the areas of low 

 water-level with those areas where there is a considerable 

 thickness of Tertiary beds, or where the top of the Chalk is a 

 considerable depth below the surface. A great thickness of 

 cover tends to check the rate of inflow of the water from the 

 chalk outcrops, and this, added to the lowering of the water- 

 level by pumping, has produced especially heavy falls in 

 certain areas. Moreover, the Chalk, so often thought extremely 

 permeable, is only so under pressure ; and if the head of 

 water in the formations above the Chalk be reduced, as, for 

 example, by the fall in the water level, the passage of water 

 through the Chalk becomes increasingly difficult. The low 

 water-level extends much farther to the north of the Thames 

 than the south. This appears to be due to three main causes. 

 There is a much thicker and more extensive cover of imperme- 

 able strata (London Clay) in the north than in the south. 

 This not only prevents rain-water from entering the Chalk, 

 but also tends, by reason of its weight, to hinder the flow of 

 water from the north towards the central areas. Then, again, 

 the intake areas of Chalk are more distant on the north than 

 on the south. Furthermore, the existence of the deep valleys 

 of the Lea and Colne transverse to the general direction of 

 underground water flow towards London, still further depletes 

 the supply on the north as compared with the south. In the 

 latter, not only are the intake areas nearer, but they are not 

 traversed by valleys in which part of the flow towards London 

 might be abstracted by springs. 



IGNEOUS QUARTZ.— The theory of the probable direct 

 igneous origin of some quartz veins and masses receives 

 support from observations made by Dr. J. Ball in South- 

 Eastern Egypt. According to the memoir, " Geography and 

 Geology of South-Eastern Egypt" (1912), this area, covering 

 twenty-two thousand square miles, consists principally of 

 ancient metamorphic and igneous rocks, with a few patches 

 of Nubian sandstone (Cretaceous), and gypseous limestone 

 (Miocene?). The igneous rocks include great masses of 

 granite and serpentine, with subordinate syenite, diorite, 

 and gabbro. In addition several masses of quartz-rock of 

 igneous origin have been found. The most conspicuous of 



these occurs as a group of three hills known as Marwot 

 Elemikan (Marwa = Arabic for quartz). They consist 

 almost entirely of white quartz, and from their dazzling white 

 colour form landmarks visible at a great distance. The only 

 other mineral present is white mica, of which a few spangles 

 occur in the outer portion of the mass. These three bosses, 

 together with another on the south side of the Wadi Khoda, 

 penetrate granite. 



Of quartz veins and dykes the best-known example in 

 south-eastern Egypt is a great dyke forming the backbone of 

 a ridge known as Erf el Fahid. This dyke is intrusive in 

 schists, is at least ten metres wide, and can be traced for two 

 kilometres. A similar dyke occurs in the ridges to the south- 

 west of Erf el Fahid. 



Dr. Ball considers that the shape of these masses of quartz, 

 their close association with aplites and pegmatites in at least 

 one locality, the absence of ores and of minerals due to 

 deposition from solution and the occasional presence of mica, 

 all point to igneous origin. He considers them as the final 

 ultra-acid products of the differentiation of a granitic magma. 



MICROSCOPY. 



By F.R.M.S. 



QUEKETT MICROSCOPICAL CLUB. — March 25th, 

 Mr. G. T. Harris presented a type collection, numbering 

 seventy-two preparations, of British Hydrozoa. 



Messrs. Heron-Allen and Earland read a paper " On some 

 Foraminifera from the southern area of the North Sea, 

 dredged by the Fisheries cruiser ' Huxley'." The work was 

 undertaken with the view of determining the distribution of 

 Saccammina sphaerica Sars. and Psammosphaera fusca 

 Schulze, in the area mentioned. Material was examined 

 from three stations far to the north-east of the Dogger Bank 

 near the Great Fisher Bank, and from three stations in the 

 belt of deep water lying to the west of the Dogger, close in to 

 the Northumberland coast. The first-named species was 

 found to occur only at two of the inshore stations, but the 

 second species was found in all the dredgings except that from 

 one of the inshore stations. The authors' first intention was 

 to examine the material solely with regard to the presence or 

 absence of these two species, but so many other species were 

 noticed that an exhaustive list was prepared containing one 

 hundred and thirty-seven species, many of which are first 

 records for this area, and were given in detail in a table 

 appended to the paper. A number of excellent lantern slides 

 from photomicrographs of some of the more interesting forms 

 was shown. 



Mr. D. Bryce read a paper on " Five new species of 

 Bdelloid Rotifers." Four belonged to the genus Habrotrocha 

 and the fifth to Callidina. They are fully described and 

 figured in the current (April) issue of the Club's Journal. 



THE MECHANICAL CONSTRUCTION OF THE 

 MICROSCOPE. — Although the optical parts of the microscope 

 have been greatly improved in recent years, the mechanical 

 construction of the instrument has not advanced in proportion. 



The books written on the subject, which have come under 

 the writer's notice, have little to say on general construction, 

 but confine themselves to descriptions of the models by- 

 various makers. 



The principal faults, in the writer's opinion, are that most 

 microscope stands are built on too small a scale and that 

 there is an unnecessary amount of patchwork in the 

 assembling of the various parts. For some purposes a 

 portable instrument is no doubt desirable, but for general 

 work ample room for manipulation, for the use of low power 

 objectives, and various accessories, is a most important 

 feature. 



The optical equipment of the microscope need not here be 

 described in detail ; it is sufficient to point out that it consists 

 of two divisions, namely, the upper or magnifying portion, 

 consisting of the ocular and objective, and the lower or 

 illuminating portion, consisting of the condenser, and mirror, 



