LOWER AND UPPER GREENSAND OF THE SOUTH OF ENGLAND. 453 
Fig. 17. A transverse microscopic section of a spicule in which the silica is 
crystalline and exhibits a radiated fibrous structure.—From chert at 
Petworth. X 1 50 diameters. 
Figs. 18, 186. Free discs or globules of colloidal silica.—From soft malm rock at 
Farnham. X 560 diameters. 
Figs. 19, 19e. Discs or globules of colloidal silica, showing an indefinite nucleus, and 
faintly-marked radiating striae. In 19<i, e, the radiations start from a 
central point.—From loose powdery material in cavity in chert, near 
Warminster. X 560 diameters. 
Fig. 20. A single coccolith from the Atlantic ooze for comparison with the siliceous 
globules. X 560 diameters. 
3 N 
MDCCCLXXXV. 
