322 



POTSTONES AT HORSTEAD. 



[Ch. XVII. 



Southern Hemisphere likewise, as Captain Maury observes, in lat. 13° 

 S., long. 16° E., for example, siliceous Diatomacese and sponge spicules 

 are the predominant forms instead of calcareous Rhizopods. 



If it be asked how the Diatomacese above alluded to can obtain a 

 constant supply of silex in solution, I may remind the reader of the 

 decomposition of felspathic rocks mentioned above (p. 42) as a copi- 

 ous source of that mineral. Almost all the great rivers which flow 

 into the ocean must contain some of it, and springs charged with 

 silex in solution must rise up in many parts of the bed of the ocean 

 as they do on dry land. 



Dr. Buckland endeavored formerly to account for the recurrence, at 

 so many distinct levels, of beds of nodular or tabular flint in the chalk, 

 by supposing the periodical accumulation of widely extended layers 

 of mud, made up of calcareous and siliceous matter. When a stratum 

 Ave or six feet or more in thickness had accumulated, its partial con- 

 solidation took place, during which the heavier silex sank to the bot- 

 tom, forming nodules, or, if it was in sufficient quantity, continuous 

 layers.* But the thickness of the masses of chalk intervening be- 

 tween some of the strata of flint has always made this hypothesis 

 somewhat unsatisfactory, although such segregation of siliceous matter 

 helps us to conceive how isolated and scattered flinty nodules may 

 sometimes be formed in the midst of a calcareous matrix. To explain 

 a regular succession of flinty layers, Ave must seek out some intermit- 

 tent action, favoring alternately the deposition of calcareous and siliceous 

 matter. Many centuries would probably be required for the growth of 

 microscopic organisms sufficient in quantity to form abed of white chalk 



From a drawing by Mrs. Gmin. 

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



Geol. Trans., First Series, vol. iv. p. 413. 



