THE MATERIALS OF STRATA. 41 



in most cases, if not always, the minute particles 



of the siliceous skeletons of sponges, some of 



which resembled the EuplecUtta speciosa from 



metimes known as \ enus's flower 



r 







■r 



\ 



Fio g Ripple-marked sandstone f 1 m Rockslntha 



J my. 



basket. The spiculae which form the skeletons 

 of such 5, dissolved by the water draining 



through the rock, furnished a cement which is 

 deposited in the same way as the cement of cal- 

 s - mdstones. Infiltration of this silica 

 frequently builds, within the sandstone, compact 

 layers o( a flinty rock, known as chert. Such 

 layers are found in the Lower Greensand, the 

 Upper Greensand, and especially in the Carbon- 

 iferous Limestone. In the S. E. of England the 

 sands and sandstones, secondary and tertiary, are 

 often coloured green, with the mineral glaucon- 

 ite. As a rule sands are red, yellow, or brown, 



