100 SECTIONAL ADDRESSES. 



Although sections of many cherts disclose the presence of siliceous 

 organisms, this is not generally the case, and although the absence of 

 such organisms is by no means a conclusive argument against their former 

 presence, yet it does not seem probable that they can have supplied the 

 immense quantities of silica that are so often met with. Hence inorganic 

 silica must sometimes be called on. 



As regards the period of chert formation, the best case for contempo- 

 raneous direct deposition is that aiJorded by the laminated chert of 

 group (c). 



Penecontemporaneous cherts in the sense defined above are illustrated 

 by those interbedded with contemporaneous dolomites, while the irregular 

 nodular cherts of the South-west Province and many other areas (group a) 

 are due to replacement and may sometimes be penecontemporaneous, but 

 in many cases are probably subsequent. The same is probably the case 

 with many of the cherts of group (b). Difference of opinion exists regarding 

 the group (b) cherts of the Midlands and N. Wales, the bulk of which, 

 according to Mr. Sargent, are due to contemporaneous deposition. 



