244 POTSTONES AT HOKSTEAD. [Cn. XVII 



transverse diameter, placed in vertical rows, like pillars at irregular dis- 

 tances from each other, but usually from 20 to 30 feet apart, though some- 

 Fig. 252. 



From a drawing by Mrs. Gunn. 

 View of a chalk pit at Horstead, near Norwich, showing the position of the potstones. 



times nearer together, as in the above sketch. These rows did not ter- 

 minate downwards, in any instance which I could examine, nor upwards, 

 except at the point where they were cut off abruptly by the bed of 

 gravel. On breaking open the potstones, I found an internal cylindrical 

 nucleus of pure chalk, much harder than the ordinary surrounding chalk, 

 and not crumbling to pieces like it, when exposed to the winter's frost. 

 At the distance of half a mile, the vertical piles of potstones were much 

 farther apart from each other. Dr. Buckland has described very similar 

 phenomena as characterizing the white chalk on the north coast of An- 

 trim, in Ireland.* 



FOSSILS OF THE UPPER CRETACEOUS ROCKS. 



Among the fossils of the white chalk, echinoderms are veiy numerous ; 



Fig. 25a 



AnanchyUs ovatus: White chalk, upper and lower. 



a. Side view. 



b. Bottom of the shell on which both the oral and anal apertures are placed ; 



the anal being more round, and at the smaller end. 



* Geol. Trans., First series, vol. iv. p. 413, "On Paramoudra," &c. 



