Cn. XVII. ] CHALK FLINTS. 243 



chalk of England and France there are no proofs of sand, shingle, and 

 clay having been accumulated contemporaneously even in European seas. 

 The siliceous sandstone, called " upper quader" by the Germans, overlies 

 white argillaceous chalk or " plancr-kalk," a deposit resembhng in com- 

 position and organic remains the chalk marl of the English series. This 

 sandstone contains as many fossil shells common to our white chalk as 

 could be expected in a sea-bottom formed of such different materials. It 

 sometimes attains a thickness of 600 feet, and by its jointed structure and 

 vertical precipices, plays a conspicuous part in the picturesque scenery of 

 Saxon Switzerland, near Dresden. 



Chalk Flints, — The origin of the layers of flint, whether in continuous 

 sheets or in the form of nodules, is more difficult to explain than is that 

 of the white chalk. I^o such siliceous masses are as yet known to ac- 

 company the aggregation of chalky mud in modern coral reefs. The 

 flint abounds mostly in the uppermost chalk, and becomes more rare or 

 is entirely wanting as we descend ; but this rule does not hold universally 

 throughout Europe. Some portion of the flint may have been derived 

 from the decomposition of sponges and other zoophytes provided with 

 siHceous skeletons ; for it is a fact, that siliceous spiculse, or the minute 

 bones of sponges, are often met with in flinty nodules, and may have 

 served at least as points of attraction to some of the siliceous matter when 

 it was in the act of separating from chalky mud during the process of 

 solidification. But there are other copious sources before alluded to, 

 whence the waters of the ocean derive a constant supply of silex in solu- 

 tion, such as the decomposition of felspathic rock (see p. 42), also min- 

 eral springs rising up in the bed of the sea, especially those of a high 

 temperature ; since their waters, if chilled when first mingling with the 

 sea, would readily precipitate siliceous matter (see above, p. 42). Never- 

 theless, the occurrence in the white chalk of beds of nodular or tabular 

 flint at so many distinct levels, implies a periodical action throughout 

 wide oceanic areas not easily accounted for. It seems as if there had 

 been time for each successive accumulation of calcareo-siliceous mud to 

 become partially consolidated, and for a rearrangement of its particles 

 to take place (the heavier silex sinking to the bottom) before the next 

 stratum was superimposed ; a process form.erly suggested by Dr. Buck- 

 land.* 



A more diflicult 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 ordinary 

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

 extensive range of quaiTies then open on the river Bure, near Horstead, 

 about six miles from Norwich, which afforded a continuous section, a 

 quarter of a mile in length, of white chalk, exposed to the depth of 26 

 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 



* Geol. Trans^ First series, vol. iv. p. 413, 



