Ch. XVII.] PEBBLES IX CHALK. 323 



several feet, and sometimes yards in thickness. We may imagine that 

 after a lapse of many years or centuries, changes took place in the 

 direction of the marine currents, favoring at one time a supply in the 

 same area of siliceous, and at another of calcareous matter in excess, 

 giving rise in the one case to a preponderance of Globigerinse, and in 

 the other of Diatornacese. 



A more difficult enigma is presented by the occurrence of certain 

 huge flints, or potstones, as they are called in Norfolk, occurring singly, 

 or arranged in nearly continuous columns at right angles to the ordi- 

 nary and horizontal layers of small flints. I visited, in the year 1825, 

 an extensive range of quarries then open on the Eiver Bure, near Hor- 

 stead, about six miles from Norwich, which afforded a continuous sec- 

 tion, a quarter of a mile in length, of white chalk, exposed to the 

 depth of twenty-six feet, and covered by a thick bed of gravel. The 

 potstones, many of them pear-shaped, were usually about three feet 

 in height and one foot in their traversed diameter, placed in ver- 

 tical rows, like pillars at irregular distances from each other, but 

 usually from 20 to 30 feet apart, though sometimes nearer together, 

 as in the above sketch. These rows did not terminate 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 cyhndrical nucleus of pure 

 chalk, much harder than the ordinary surrounding chalk, and not 

 crambling 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 simi- 

 lar phenomena as characterizing the white chalk on the north coast 

 of Antrim, in Ireland.* 



These pear-shaped masses of flint often resemble in shape and size 

 the large sponges called Neptune's cups (Spongia patera, Hardw.), 

 which grow in the seas of Sumatra ; and if we could suppose a series 

 of such gigantic sponges to be separated from each other, like trees in 

 a forest, and the individuals of each successive generation to grow on 

 the exact spot where the parent sponge died and was enveloped in cal- 

 careous mud, so that they should become piled one above the other in 

 a vertical column, their growth keeping space with the accumulation 

 of the enveloping, calcareous mud, a counterpart of the phenomena of 

 the Horstead potstones might be obtained. 



Single pebbles in chalk. — The general absence of sand and pebbles 

 in the white chalk has been already mentioned ; but the occurrence 

 here and there, in the south-east of England, of a few isolated pebbles 

 of quartz and green schist, some of them 2 or 3 inches in diameter, has 

 justly excited much wonder. If these had been carried to the spots 

 where we now find them by waves or currents from the lands once 

 bordering the cretaceous sea, how happened it that no sand or mud 



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



