452 



NATURE 



{^April i„ 1872 



meablc strata ; and careful measurements have proved that the 

 discharge at Paris is also equal to about one-third of the rainfall. 

 The exact proportion of the rainfall passing into the different 

 permeable strata, and given out again in the form of springs, has 

 yet to be accurately determined. Mr. Harrison estimates it in 

 the Thames basin at about one-sixth of the rainfall. 



In districts where impermeable strata predominate, the total 

 water delivery, therefore, will be greater ; but it follows close 

 upon the rainfall ; whereas, where the permeable strata pre- 

 dominate, a large portion of the rainfall is stored in the hills, 

 and its delivery is th-reby spread over a greater or lesser period 

 of time, according to the dimi-nS'ous of tho^e hill-s. This is well 

 evempli'ied in the case of ihe basins of the Thames and the 

 Severn, widch latter is formed in large part by the slate rocks of 

 Wiles The former has an area above Kingston of 3.670 square 

 miles, with an annual rainfall of 27 inches; whereas that of the 

 latter above (iloucster has an area of 3,890 miles, with an 

 average rainfall of prob.rbly not less than 40 inches, and the mean 

 daily discharge for the year is for the Thames of 1,250,000,000 

 gallons, and (or the Severn about 1,600,000,000 gallons. Yet 

 the summer discharge of the Thames averages 688,700,000 

 gallons daly, against 297,599,040 gallons of the Severn ; and 

 while ihe minimum discharge of the 'I'hames in the driest seasons 

 never falls helow 350,000,000 gallons, thaf of the Severn falls 

 below 100,000,000 gallons. Again, in the case of the Lea, 

 where there is a still larger proportion of permeable strata, the 

 daily discharge at Bro.xbourne for the year is, according to Mr. 

 Beardmore, 108.000,000 gallons, while for the summer months 

 it remains as high as 71,000,000, and in the driest seasons does 

 not fall below 42,000,000 gallons. 



Let us now look at one of the geological questions dependent 

 upon the solvent action of the water on the strata it traverses. 

 The analyses, maiie for the Commission by Drs. Frankland and 

 Odling, of the waters of the Thames and its tributaries in the 

 Oo itio and Chalk area, show that every 100,000 parts or grains 

 of rainwater has taken up a quantity varying from 25 "58 to 

 32 '95 grains of solid residue, or an average of 29 "26, which 

 is equal to 20*48 parts or grains per gallon ; another analysis 

 of the Thames water at Ditton gives 207S grains per gallon 

 of solid residue. It was also shown by Drs. Letheby and 

 Odling and Prof. Abel that the unfiltered waters of the Thames 

 Companies, which take their supplies above Kingston, con- 

 tained 20 82 of solid residue, If from the average of 20*6S we 

 deduct I '68 grain for organic and suspended matter, we have 

 19 grains of morganic residue for every gallon of water flowing 

 past Kingston. This is of course apart from the sediment 

 cariied down in floods. The ordinary monthly analyses, con- 

 ducted by the same eminent chemists during the course of sever.al 

 past years, show that this quantity is liable to very little varia- 

 tion, the only difference being that it is somewhat larger in 

 winter and less in summer. 



Some general estimates have already been made by Profs. 

 Ramsay and Geikie of the quantity of mineral matter carried 

 down in solution by the Thames ; but the more exact data sup- 

 plied to the Commission enable us to make some additions to 

 previous results. Taking the mean daily discharge of the 

 Thames at Kingslon at 1,250 million gallons, and the salts in 

 solution at 19 grains per gallon, the mean quantity of dissolved 

 mineral matter there carried down by the Thames every twenty- 

 four hours is equil to 3,364,2861115. or 1502 ions, or 548,230 

 tons annually. Of this daily quantity about two-thirds, or 1,000 

 tons, consist of carbonate of lime, and 238 tons of sulphate of 

 lime, while 'imited proportions of carbonate of magnesia, Lhlo- 

 rides of .-odium and potassium, sulphate-i of soda and potash, 

 silica and traces of iron, alumina, and phosphates, constitute ihe 

 rest. If we refer a small portion of the carbonates, and ttie 

 sulphates and chloi ides chiefly, to the impermeable argillaceous 

 fo mations was! el by the rain water, we shall s'ill have at least 

 10 grains per gallon of carbonate of lim -, due to the Creaceous 

 and Oolitic strata and Marlstone, the superfici-il area of which, 

 in the Tnames basin above Kingston, is es imate-d by Mr. Har- 

 rison at 2,072 square miles. Therefore the annual qu-^nticy of 

 carbonat-^ of lime carried away fro.n this are^ bv tlie Thames is 

 29 < 905 tons, or 797 tons daily, which gi.es 140 tms removed 

 yeariy frjm each square m le ; or extending the cjlcu'ation to a 

 century we have 14 000 tons removed from each mile of surface. 

 Taking a ton of cialK as equal to 15 cu'tic feet, this is equal to 

 a remov,-.l of -rxhj o' ^n inch from the surlacc in the course of a 

 c n'li-v, so thit in the course of 13,200 y.ars a quantity equil to 

 a tnickness of about one foot would be removed from our Cha'k 

 aad Oolitic districti. 



I had some faini Inpe that this wear might furnish us with a 

 rough approximate measure of time in reference to some of the 

 phenomena connected with the f,)uatemary period ; but we are 

 not in a position to apply it. Those curious funnel-shaped cavities, 

 called sand and gravel-pipes, so common in many chalk-districts, 

 are the result of slow solution of the chalk by water at particular 

 spots, whereby the superincumbent sand and gravel have been 

 let down into the cavity so produced. Some of Ihem are but a 

 few feet deep, while others attain dimensions of So feet in depth 

 by 15 to 20 feet in diameter at top, tapering irregularly to a 

 poin^ at bottom. It is, however, evident from the variation in 

 size that the wear has been unequal ; and it is also clear that the 

 surface-waters have been conducted through these particular 

 channels, w'here they existed, to the underground water level, in 

 preference to passing through the body of the chalk, so that Ihe 

 latio of wear at these pjints is in excess. Nor can I see at pre- 

 sent how otherwise to apply this measure. If it were possible 

 to find a spot where the exposed surface of the chalk has been 

 worn uniformly, and, from the quantity of flints left after the 

 removal of the chalk and the known distance apart there of the 

 seams of flint, to determine the number of feet or inches re- 

 moved, we might have a base to proceed upon, provided all the 

 quantities remained constant. But such is not the case. Also, 

 although the annual rainfall in the Thames now averages 27 

 inches, and has probably not varied much from this amount 

 during the present period, it was evidently much greater during 

 the Quaternary period ; for I have elsewhere shown that, in the 

 .South of England and North of France the rivers of those areas 

 with the same catchment-basins were of much greater size 

 than at present ; and Mr. W. Cunnington had before pointed 

 out the same fact in the upper part of the basin with respect 

 to some of the rivers of Wiltshire. M. Belgrand has made 

 an attempt to estimate this quantity with reference to the 

 Seine and its tributaries, and he arrives at the conclusion 

 that, during the Quaternai'y (or, as he considers it, the Glacial) 

 period, the rainfall was s > heavy, that the discharge of the 

 river was from 20 to 25 times greater than at present. I do 

 not altogether concur in this view, but I can conceive that 

 our rivers formerly were of five or six times the size they 

 now are. This is an important element to be considered in 

 all questions bearing on the denudation of land- surfaces. 



There is yet another point which, although not in our direct 

 field of research, yet depends so essentially upon the geological 

 conditions we have discussed, and is one, in a public point 

 of view, of such paramount importance, that I will, \\'ith your 

 permission, say a few words on the subject. In an uninhabited 

 country, the rain passes through the soil and issues as springs, 

 bearing with it a certain proportion of mineral matter, and only 

 traces of such organic matter as existed on the surface. This 

 would be solely of vegetable origin, and the proportion would be 

 in most cases very small. As man appeared, those conditions 

 would be at first but little altered, for animal matters exposed on 

 the surface rapidly decay and pass away in a gaseous form ; but 

 with increasing civilisation and fixed residences the necessity of 

 otherwise getting rid of all refuse would soon be t'elt. I have 

 shown how population followed the range of shallow permeable 

 strata and the course of valleys, so as to obtain readily that in- 

 dispensable necessity of life, a sufficient w.iter supply. But with 

 the art of well-digging it soon became apparent that, let the well 

 be carried down bu' half way to ihe level of ground-springs, it 

 would remain dry, and ihat then, so far from holding water, any 

 wa er now poured into it would pass through the porms strata 

 down to the water-level beneath, keeping the shallower well or 

 pit constantly drained. So convenient and ready a means of 

 getting nd of all refuse liquids was not neglected. Whilst on 

 one side ot the house a well was su 'k to the ground-springs, at 

 a depth, say, of twenty feet, on ihe other side a dry well was 

 sunk lO a depth of ten feet, and thi, was made the receptacle of 

 house-refuse and sewage. The sand or gravel acting a-, a filter, 

 the minor solid matter lemained in the dry well, while ihe major 

 liquid portion passed chtojgh the permeable stratum and went to 

 feed the unlerlying springs. Wh.it was done in one hou^e was 

 d-'iie in the man\ ; and -Atljat was done by our rude ancestors 

 Centuries back his continued to be the practice of tneir more 

 cuUiViteJ descendants to t le present day, witn a persistency in 

 the method only to be attribu'ea to the ignorance o( the existence 

 of such a state of things am.jiig the masses, and to the ignorance 

 of ihe leal conditions and actual results of perpetuating such an 

 evil — an evil common alike to the cutt.ages of the poor and, with 

 few exceptions, to ihe mansions of the rich. 



