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the clifis arc somotliing like greased planks uudcr the feet of the 

 unfortunate geologist, who may think of making his retreat by this 

 line of route to Dover. The chalk may be considered to form the 

 larger portion of Kent, for the tertiary beds, as before stated in the 

 section of the physical geography of the county, only occupy hol- 

 lows or depressions on the surface of the chalk. In a paper published 

 with the Annual Report for 1878, page 33, I drew attention to the 

 organic origin of chalk, for there is reason to suppose that the whole 

 of the chalk comprising the North and South Downs, and extending 

 from the south of Dorchester to Flamborough Head, Yorkshire, and 

 of more than seven hundred feet in average thickness has passed 

 through the bodies of living creatures. Some chalk contains 

 ninety-eight per cent, of carbonate of lime, tlie remainder being 

 alumina and silica, with small quantities of oxide of iron. In the 

 lower chalk, which is about four hundred feet in thickness, 

 there are no. or few, flints, but in. the upper chalk, about thi*ee 

 hundred feet iu thickness, the bauds of nodular flint occur, so 

 familiar to everyone acquainted with this district, forming the 

 long lines of black nodules which, by their contrast with the 

 white rock that they are imbedded in, constitute a very striking 

 feature in some of the sea cliffs, notably so at the Needles on the 

 coast of the Isle of "Wight, where these lines are almost vertical, 

 the chalk having been turned on end long after its deposition in 

 the depths of the cretaceous sea. In the Dover cliffs the dip to 

 the north-east is so small that they appear horizontal. Many 

 conjectures have been made as to the origin of flint. One 

 scarcely deserving notice was that they were of meteoric origin, 

 and so fell from the skj^ in a state of igneous fusion, and settled 

 down into the sea when the chalk was forming at the bottom. 

 This, so soon as the nature of flint was enquired into, 

 became untenable, as they were found to contain delicately 

 organised structures and fossils, that evidently had not been 

 exposed to the extreme temperature required to vitrify 

 silica. Dr. Bowerbank, whose researches in sponges ren- 

 dered their structure so familiar to him, came to the 

 unhesitating conclusion that the flint, nodules were once sponges 

 growing at the bottom of the sea, and ihat the gelatinous matter 

 known as sarcode, which covered their network skeletons, became 

 replaced by silica, so that we may regard these bodies as petrified 

 sponges. The resemblance of them to an ordinary bath sponge is 

 sometimes quite striking, the large holes fre(]^uently passing through 

 them, and the mass occasionally holding in its embrace a shell, are 

 precisely that which is observable iu either case. Thin sections of 

 flint also show the spongy structure, and added to this, the peculiar 

 spicules of sponges arc frequently found in flint. That such struc- 

 ture is discernable in the chalk does not militate against this theory, 

 it only shows that there were many sponges that did not become 



