34 OUR COMMON BRITISH FOSSILS. 



form the chert. This is frequently the case in the chalk. 

 Radiolarians and other minute siliceous organisms 

 are present in great abundance, diffused through the 

 chalk where there are no flint-bands or nodules ; 

 whereas the chalk is always freest from them where 

 such nodules regularly occur. 



In this manner the student perceives that the 

 presence or absence of some of the most minute of 

 animal remains may determine the petrological 

 character of the great rock masses which build up 

 the continents of the globe. 



