CHALK. 399 



segregation.^ The formation of flints may in some instances have taken place 

 during the consoUdalion of the Chall<, and have been induced or accelerated by the 

 pressure of continual dejiosits, for it has been stated by Mr. Sorby that mechanical 

 pressure increases the solubility of most substances. Dr. Wallich, who insists that 

 flints in their method of origin may as truly be considered organic products as the 

 Chalk itself, has given an explanation of the flint-layers which makes their regularity 

 more intelligible than it has been hitherto. 



The upper layer of Chalk-mud at the bottom of the abyssal ocean is the home 

 of numerous vitreous or siliceous sponges ; and it is filled with the delicate siliceous 

 root-fibres of the sponges, which bind it together. Owing to its inferior specific 

 gravity it floats, as it were, upon the calcareous mud beneath. Thus this super- 

 ficial layer contains a large per-centage of silica, not merely from the sponges 

 themselves, but from the continual subsidence of minute dead siliceous organisms, 

 which become incorporated in its mass. Finally it becomes supersaturated with 

 silica, and the first step towards the consolidation into flint is accomplished. Thus 

 in time various bands of flint and layers of calcareous mud would be formed.^ 



Paranioiidras. — In some places, as in Norfolk, the Chalk contains huge flints 

 of a cylindrical or pear-shaped form, having a central cavity passing through their 

 longer axis, which occupies a vertical position in the Chalk. These flints are 

 known as "Pot-stones" or Paramoudras. The term Paramoudra has been a 

 source of much perplexity. It was introduced from Ireland by Dr. Buckland, who 

 first described these curious flints found on the coast of Antrim and at Moira, in 

 county Down.^ Mr. H. Norton suggests that the word is an anglicized form 

 of the Gaelic and Erse words for "sea-pear," most likely used in the plural, feura 

 mtiireach.^ 



The most celebrated exposure of Paramoudras was in a pit at Horstead, on the 

 river Dure, of which a drawing made by the late Mrs. Gunn in 1838 has been 

 published in Lyell's Elements of Geology. The pot-stones, many of them pear- 

 shaped, were usually about three feet in height and one foot in their transverse 

 diameter, placed in vertical rows, like pillars at irregular distances from each other, 

 but usually from twenty to thirty feet apart, though sometimes nearer together, as in 

 the above-mentioned sketch.^ 



The Horstead pit is now closed, but paramoudras occur occasionally in most 

 of the pits around Norwich, at Trowse, Catton, and Whitlingham. On the whole, 

 many more instances might be cited where the paramoudras occur in an isolated 

 position, or irregularly, than where they occur vertically one above another. 

 So variable too are the shapes assumed by large flints that a series might be 

 obtained to show transitions from the most symmetrical paramoudra to a large 

 irregular and tabular mass of flint having one or two openings or cavities in it. 

 Mr. T. G. Bayfield, of Norwich, had in his collection a paramoudra having 

 a double cavity, and there is a similar "double paramoudra" in the Ipswich 

 Museum. Lyell remarked that some of these pear-shaped flints resemble in 

 shape and size the large sponges called Neptune's Cups {Spongia patera), which 

 grow in the seas of Sumatra. Ehrenberg, however, attributed their origin to the 

 sinking of silica (deriving its origin from organic remains), by its own gravity, in a 

 funnel shape, through a more solid but naturally still soft layer of chalk. ^ This 

 explanation is not inconsistent with the account given by Dr. Wallich of the sheets 

 of organic silica on the ocean-bed. 



In many places we find flints which exhibit a white coating surrounding a 

 nucleus of dark compact flint. This white coating often occurs as a thin film on 

 flints directly taken out of the Chalk, but in specimens derived from Drift deposits, 

 the coating has often eaten its way far into the interior of the flint, so that in 

 many cases but a tiny nucleus remains to indicate the original character of the 



^ C. Reid, Geol. Cromer (Geol. Survey), p. 4 ; see also Penning and Jukes- 

 Browne, Geol. Cambridge, p. 70. 



- Q. J. xxxvi. 68 ; Ann. Nat. Hist. 1881, pp. 162, 261. 



^ T. G. S. iv. 413. * Proc. Norwich Geol. Soc. i. 132. 



^ Lyell, Elements of Geology, 1865, p. 321 ; see also Geol. Norwich (Geol. 

 Surv.), p. 24. 



^ Ann. Nat. Hist. 1839, ii. 162. 



