June 7, 191 7] 



NATURE 



28 



of this work formed three lectures which were 

 recently delivered at the Royal Institute of Public 

 Health, and the fourth chapter is upon the subject 

 of sterilisation. This term is restricted in its 

 application to the destruction of microbes causing 

 epidemic water-borne disease ; and the methods 

 set out embrace the "excess lime " treatment 

 (which is Dr. Houston's own suggestion) and the 

 "chlorination " of water. 



Some persons may consider that Dr. Houston 

 takes too sanguine a view with regard to the 

 safety of rivers as sources of water-supply. There 

 can be no doubt, however, that the large amount 

 of exp>erimental work he has undertaken, notably 

 that which illustrates the prime value of storage 

 as a means of reducing the risk of water-borne 

 disease, justifies his sanguine views. After all, 

 where London has succeeded other towns can 

 also succeed, always provided that in these other 

 towns the same careful working is maintained 

 by a well-trained personnel as is the case with 

 London, and that a similar constant and scientific 

 control of the state of the water is maintained. 

 The danger is that these provisions may not 

 always be made. 



The American Indians North of Mexico. By 

 W. H. Miner. Pp. x+169. (Cambridge: At 

 the L^niversity Press, 191 7.) Price 35. net. 



The literature connected with the North American 

 Indians is so extensive that a readable summary 

 of it in a popular form was much needed for the 

 use of European anthropologists commencing 

 the study. In America, particularly among the 

 descendants of the hardy frontier men, the ques- 

 tion is attracting increased attention. This want 

 is well supplied in this book. The advanced 

 student will depend not only on the classical 

 works of Bancroft, Schoolcraft, and Catlin, but 

 also on the monographs published by the Bureau 

 of American Ethnology and other societies which 

 have been summarised, with the addition of much 

 new matter, by Mr. F. W. Hodge in his excel- 

 lent " Handbook of American Indians North of 

 Mexico." The questions connected with the 

 origin of these tribes still form the subject of 

 controversy. The writer remarks that the 

 general consensus of opinion during the last cen- 

 tury is to the effect that, "with the exception of the 

 Eskimo, the natives of America are wholly of one 

 race and descendants from early emigrants from 

 north-eastern Asia, and especially of Mongolian 

 stock." But the movements of these people 

 within the American continent have as yet not 

 been definitely settled. The importance of 

 linguistics for the settlement of these problems 

 is fully recognised. The book, after a summary 

 account of the environment, discusses the 

 sociology of the tribes, and gives details of some 

 members of the Plains Indians and those of the 

 south-west. The culture of the Pueblos forms the 

 subject of an interesting chapter. There is a 

 good bibliography, and the book may be com- 

 mended as a satisfactory popular introduction to 

 the study of a remarkable people. 

 NO. 2484, VOL. 99] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



The Origin of Flint. 



May 1 ask you to publish the following notes on 

 the origin of flint? I understand that a discussion of 

 the subject has been initiated by those interested in 

 the chemical and physical aspects of geology, and 

 think that the facts cited below may be of interest at 

 this moment. 



I understand by the term " the origin of flint " an 

 account or reasonable explanation of the formation of 

 the nodules of black flint which occur so abundantly 

 in stratified layers in the Upper Chalk of this country. 



Some seventy years ago the view was put for- 

 ward by the well-known naturalist Bowerbank — who 

 was a special student of the sp>onges — that the flint 

 nodules of the chalk were formed in situ in the depths 

 of the sea by the silicification of sponges which already 

 contained abundant siliceous spicules, and were, as it 

 were, solidified by attracting to themselves additional 

 silica from the sea-water. Silicification of wood — as in 

 the case of some wooden piers erected in shallow seas 

 — was, known. The segregation of silica by the 

 attraction for it of organic matter was a recognised 

 fact. Similar segregation and formation of "concre- 

 tions" of other chemical substances by other attractive 

 nuclei was recognised. Thus lumps or small masses 

 of clay were shown to have the power of attracting 

 phosphate of lime, and so to give rise to those 

 "phosphatic" nodules found at the base of the Red 

 and Coralline Crag, and also at the base of the Cam- 

 bridge Greensand, and in other positions where the 

 ':)ones cf animals were accumulated and furnishei 

 phosphate of lime, which was first dissolved by the sea- 

 water and then removed from it and held by the clay 

 nodules. 



From time to time other views were put forward 

 as to the formation of the flint-nodules of the chalk 

 after the deposit of the chalk yet whilst it was still 

 beneath the sea and permeated by sea-water. It was 

 held that the organic remains deposited in strata in 

 the chalk sea-bottom exercised an attractive influence 

 on the silica dissolved in sea-water, and so led to the 

 replacement of the organic remains by solid silica. 

 Later it became fairly certain that, as is the case with 

 the Atlantic ooze, the chalk deposit contained origin- 

 ally about 10 per cent, of colloid silica in the form of 

 spicules and skeletons of minute organisms, and it was 

 held that this silica was dissolved by the permeating 

 sea-water (whilst the chalk was still beneath the sea), 

 and was then separated and deposited in the cavities 

 occupied by sponges and other organic remains in 

 stratified layers in the chalk. 



It is difficult enough to find a parallel for this sup- 

 posed deposit when the solid, fairly (though not com- 

 pletely) homogeneous character of the black chalk 

 flint is borne in mind. It is remarkable that the flint 

 deposited in these cavities shows little or no trace of 

 onion-like concentric layering, such as characterises 

 the agates formed in geodes of igneous rock. We also 

 are met with a striking fact, namely, that the black 

 flint is apparently micro-crystalline in structure, and 

 that its behaviour when "w-eathered" is such as to 

 lead to the inference that, although homogeneous to 

 the unaided eye, it yet consists of minute particles of 

 quartz (that is to say, crystalline silica of the same 

 nature as rock-crA-stal) cemented by colloid silica, 

 which latter dissolves to a certain extent in alkaline 



