98 SECTIONAL ADDRESSES. 



The two chief problems which confront a student of the cherts in any 

 area are to determine in the first place the source of the silica, and in the 

 second place to obtain proof as to its period of origin, i.e. to ascertain 

 whether its deposition was contemporaneous with that of the associated 

 strata, or whether it was due to some process of replacement. 



Note.- — There are possibilities of confusion in the use of the term contempo- 

 raneous. It may be used (1) as implying direct deposition at the 

 same time as the associated strata. Or it may be used (2) in a sense 

 corresponding to that in which the term contemporaneous dolomite 

 is used, implying that the chert was produced by a metasomatic 

 change shortly after the formation of the limestone with which it is 

 associated. To avoid confusion I propose to allude to cherts of 

 type (1) as contemporaneous and those of type (2) as penecontempo- 

 raneous. The term subsequent may then be applied to cherts produced 

 by a relatively late alteration, i.e. one taking place after the consolida- 

 tion of the limestone. 



Mr. Dixon points out (by letter) that proof as to the period of origin 

 is clearest in the case of cherts associated with contemporaneous dolomite. 

 The organisms are perfectly preserved in the chert but obliterated in the 

 surrounding dolomite. Consequently chert formation preceded the bulk 

 of the contemporaneous dolomitisation. 



The problems both as to the source of the silica and as to the period of 

 its formation have attracted much attention of recent years, especially 

 in America, and have been discussed in considerable detail by Mr. H. C. 

 Sargent ^2" in his study of the cherts of the Midlands and North Flintshire. 

 As regards the source of the silica, one view is that it has a directly organic 

 origin, being derived from the tests of radiolaria,or perhaps in some cases 

 of diatoms, and from the sjDicules of certain kinds of sponges. The other 

 view regards the silica as directly deposited from solution in sea-water. 



The three types of chert may be separately considered. 



{a) The irregular nodular chert often forming impersistent bands is the 

 prevalent type in the South-west Province, and is probably the most widely 

 spread in general. I have found no radiolaria and few sponge spicules 

 in a limited number of sections of this rock from the Bristol district. Hull 

 and Hardman,^^^ and also Renard,^-^ referring probably to chert of this 

 type, consider it to be a pseudomorph of gelatinous silica after limestone, 

 and believe that the change took place when the strata were more or less 

 plastic. In the Burrington section the chert is particularly developed in 

 bands of Lithostrotion, and at Waterlip and Windsor Hill in the Mendips 

 in highly crinoidal limestone. In each case I am convinced that the 

 chert is due to replacement, and I do not see any reason to assume that 

 the limestone was still plastic when this took place. I should therefore 

 regard the chert as subsequent in the sense alluded to above. As far as 

 I know, all workers on the limestone of the South-west Province agree as 

 to the origin of the associated chert by replacement. 



120 Geol. Mag., vol. Iviii. (1921), p. 265, and vol. Ix. (1923), p. 168. 

 1=1 8ci. Trans. Roy. Dublin Soc, vol. i. (1878), pp. 71-94. 

 1" Bull. Acad. Boy. de Bel i ue 2me aer., t. 46, pp. 471-98. 



