Aprtl i„ 1872] 



NATURE 



451 



ANNUAL ADDRESS TO THE GEOLOGICAL 



SOCIETY OF LONDON, FEB. 16, 1872 



By J. Prestwich, F.R.S., President 



(Coniinued from page 433.) 



IT has been already mentioned that below a certain level perme- 

 able strata are necessarily alviays saturated and water-logged, 

 and that any additional quantity added to this constant quantity 

 cannot be held permanently. It follows that wherever, m all 

 water-bearing strata, afier allowing for any abstraction, usually 

 but comparatively small, by wells, the surplus rainfall must, 

 when the stratum is full, find its escape by natural means, ;' f. , by 

 means of springs. The power and size of these are necessarily 

 dependent upon the dimensions of the strata by which they are 

 supplied. In the gravel they are small, in the Lower Tertiary 

 sands moderate ; while in the Chalk they are very large. The 

 permanence of the spring depends on the lithological character 

 as well as on the dimensions ol the strata. Thus, in sands, where 

 the water can permeate the mass, the stores are large, and the 

 delivery moderately quick ; in Limestones, where the water is 

 confined to cracks and fissures, the delivery is quick and not 

 lasiing, though often large ; in rubbly Oolites, which are also 

 practically porous, the springs are well maintained ; while in 

 Chalk, owing to the characters before named, the water-delivery 

 is slow, and the springs are large and very permanent. 



At the same time the storage-capacity increases with the re- 

 sistance. Taking the extreme case of the Chalk, the trans- 

 mission of the rain-water is so slow, that, on the chalk hills, 'it 

 takes four or six months to pass from the surface to the line of 

 water-level at the depth of 200ft. to 300ft., so that the 

 heavy rainiall of winter is not felt in the deep springs until the 

 summer, and Mr. Beardmore estimates that the minimum eflfect 

 of a hot dry summer and autumn is not reached until at the end 

 of about sixteen months, or that the storing-power of the chalk 

 is of sixteen inonths' duration. To estimate this power, we have 

 to take the height and extent of the hills, and to note the litho- 

 Icgical characters of the permeable strata. If these latter are 

 underlaid by impermeable strata at above the level of the 

 rivers in two adjacent valleys, then the base of the under- 

 ground water-store will be coincident with the level of the im- 

 permeable strata, and its surface-line will rise, as it recedes within 

 the hill, in proportion to the resistance offered to the water's 

 escape by the character of the permeable strata, and it will thus 

 form a curve between those two points, the height of which will 

 vary in proportion to the rainfall. When, on the other hand, 

 the permeable strata continue down to a gieater or less depth 

 beneath the surface of the adjacent rivers, then, as there is no 

 underground escape for the stored water, the line of water-level 

 in those permeable strata will rise to, and be always maintained 

 at, the level of the rivers, and therefore all the additional sup- 

 plies furnished by the rain must, after traversing the interior of 

 the hills, find an escape along the bottom of the valleys, and by 

 the side or in the bed of those rivers In the dry upland valleys 

 of the Chalk and Oolites, the underground water, dammed back 

 by the streams in the nearest river-valley, passes under those 

 valleys at depths varying with the resistance offered by the 

 litholiigical character of the formation, and by the gradient of the 

 valley as it runs into the hills. 



When again, as in the case of the chalk downs and oolite 

 hills, the exterior outcrop of the permeable strata rests on im- 

 permeable strata at a bright above the river-levels, and in the 

 other direction inwards they pass below similar levels, then the 

 springs partake of the same divided character — the one smaller 

 set flowing out on the sides of the hills, and ihe stronger and 

 more lasting springs issuing, as it were, at the foot of the incline 

 on the level of the rivers. In any case, it is the distance between 

 the two points of escape that gives us one measure of storage. 

 If the distance is reckoned by miles, then the rise of the waier- 

 level may be measured by tens of feet. It is highest when both 

 the distance from the adjacent river-valleys, and at the same 

 time the height of the hills is greatest. In some instances, the 

 crown of the arch formed by it will rise to a height of from 60 ft. 

 to So ft. above its chord. 



This curve is subject to great fluctuation, varying according to 

 the seasons and amount of rainfall. Mr. Clutterbuck has shown 

 that, in the chalk hills of Hertfordshire, its height varies as 

 much as 30 ft. or 40 ft. From the crown or centre of its summit 

 it decreases at a rate varying generally from 3 ft. to 30 ft., or 

 even more, per mile to all parts of the circumference. The 



height of the arch and the breadth of the base-line, taken to- 

 gether, give therefore the head of water supplying ihe large springs 

 of the Chalk — such as those of Chadwtll, Hoddesden, Otter, 

 Carshahon, Leatherhead, Ospringe, and others. But, besides 

 these, thf-re are innumerable smaller ones, not so easily seen, 

 flowing out on the sides, or in the beds of the rivers I raversing the 

 great permeable formations, as the many along the Thames from 

 Greenhithe to Faversham, on the Upper Lea and its tributaries, 

 and on the Medway and the Darent, where they traverse the 

 chalk hills. This class of springs has especial geological 

 bearings, which we shall hereafter have occasion to dwell upon. 



The same general rules govern the springs of all the more 

 varied strata of the upper part of the Thames basin, where, in 

 place of the Cretaceous and Tertiary series, we have a series of 

 Jurassic and Liassic strata. Omitting the drift or gravel beds, 

 the following are the average dimensions, character, and super- 

 ficial areas of each of these formations in that area : — 



Strata of the Thames Basin above Wallingford 



Square 

 miles. 



Chalk (above Kingston 1047) 60 ... 1000 ... — 



Upper Greensands ... ... 62 ... 100 ... — 



Gaiilt 129 ... — ... 130 



Lower Greensands ... ... 23 ... 200 ... — 



Purbeck and Portland beds 46 ... 60 ... — 



KimiHt-ridge Clay 132 ... — ... 300 



Coral Rag and grit... ... 103 ... 40 ... — 



Oxford Clay 307 ... — ... 400 



Great and Inferior Oolites... 327 ... 450 ... — 



Iidli-r's Earth 16 ... — ... 40 



Lias 170 ... — ... 500 



But although many of these water-bearing strata are of large 

 dimensions and well stored in the upper part of the Thames 

 basin, none of those below the Gault, except the Lower Green- 

 sand, are available for a well-supply at London. The Upper 

 Greensand, so important in Wdtshire, is reduced to a few feet of 

 comparatively impermeable argillaceous sands under London. 

 The Oolitic series, so rich in springs in the district of the Cots- 

 wold Hills, have been ascertained to thin off as they range east- 

 ward ; and Mr. Hull has shown that the inferior Oolite and 

 underlying sands in particular die out. in all probability, under 

 the Oxford clay about the centre of Oxfordshire. Even apart, 

 therefore, from the discovery made at Kentish Town, we should 

 now have excluded the Oolitic series as a possible source of supply 

 to deep wells in the London district ; although, as sources of 

 springs' supplies, they contribute so important a share to the 

 maintenance of the Thames. Few of those strata are, how- 

 ever, so homogeneous as the Chalk and the London Clay. The 

 permeable formations often contain subordinate impermeable 

 clays — seams which form water-levels of more or less importance, 

 whilst the impermeable clays sometimes contain subordinate 

 beds of sand or of rock which constitute small local water- 

 bearing beds. It is for the geologist to assign its relative value 

 to each of these subordinate features, and to distinguish the 

 minor from the major sources. 



Taking the Thames basin above Kingston, there is. according 

 to Mr. J. D. Harrison, an area of 1,233 square miles of im- 

 permeable strata, and of 2,442 miles of permeable strata, and 

 the mean annual rainfall in that district amounts to about 

 27 inches. From the impermeable strata the rain flows off immedi- 

 ately as it falls, and is carried at once to sea ; whereas a large 

 portion of that which falls on the permeable strata is, as we have 

 shown, stored for a greater or lesser tinie, and discharged in 

 perennial springs. It is these which give permanence to our 

 rivers. The evidence taken before t^ie Commission showed that 

 the daily discharge of the Thames at Kingston, even in the 

 driest season after weeks without rain, never falls below 

 350,000,000 gallons, while the average for the year gives, ac- 

 cording to Mr. Simpson and Mr. Harrison, 1,353,000,000 

 gallons, or, according to Mr. Beardmore's longer observa- 

 tions, 1,145,000,000 gallons daily, the mean of 1,250,000,000 

 gallons bi-mg equal to a fall of about 8 in., or rather less 

 than one-third of the annual quantity, the other two-thirds 

 being lost by evaporation and absorbed by the vegetation. 

 This seems the proportion usual under the like general con- 

 ditions in these latitudes. M. Belgrand has shown, in "La 

 Seine," that in the upper basin of the Seine there are 

 19,390 square kilometres of impermeable, and 59,210 of per 



