THE GAULT AND UPPER GREENSAND 41 



a kind of sponge, Siphonia. It has the form of an oblong 

 bulb, supported by a long stem, with a root-like base. 

 It is often silicified, and when broken shows bundles of 

 tubular channels. 



In the chert may often be seen pieces of white or bluish 

 chalcedony, generally in thin plates filling cracks in the 

 chert. This is a very pure and hard form of silica, 

 beautifully clear and translucent. Pebbles which the 

 waves have worn in the direction of the plate are very 

 pretty when polished, and go by the name of sand agates. 

 They may sometimes be picked up on the shore near the 

 Culvers. 



