ORIGIN OF FLINT. 215 



stitute caxnelian, while when it is arrauged in differently 

 coloured bands it forms agate. Occasionally small crystals 

 of quartz occur in the hollows of flint. 



It has still to be mentioned that in some districts — and 

 more especially near Norwich — in addition to the hori- 

 -zontal layers of nodular flints, there occur in the upper 

 chalk a number of huge cup-sbaped masses of flint j)laced 

 one above another in vertical lines ; these masses being 

 locally known as "potstones," and j^resenting a remarkable 

 resemblance to certain giant sponges called Neptune's 

 cups. 



The piroportion of the flint to the chalk in the upper 

 chalk of England varies, acording to Prof. Prestwich, 

 from four to six pier cent. It is important to add that the 

 masses of nodular flint may not unfrequently be found to 

 be traversed by fractures which have subsequently been 

 reunited ; thus showing either that the substance must at 

 the time have been in a semi-plastic condition and capable 

 of reunion, or that the fracture has Ijeen united by the 

 subsequent deposition of siliceous matter. It must also be 

 mentioned that, as a rule, the white coating is confined to 

 the outer surface of the nodules, and that we do not find 

 layers of pure flint overlain with white coats and then 

 again by other similar layers ; thus indicating that the 

 formation of the white coat was the final act in the 

 development of flint. 



With these observations on the structure and mode of 

 •occurrence of flint, we are in a position to enter on the 

 more diificult subject of its origin. From the very first it 

 was recognized by all geologists that such a peculiarly 

 hard and homogeneous substance as flint, occurring in 

 irregular nodules among the pure white chalk, could not 

 have been deposited in its present condition directly from 

 the waters of the cretaceous sea ; and the problem there- 

 fore presented itself to account adequately for its mode of 

 formation. The problem soon resolved itself first of all 

 into two questions, namely, whether the silica was originally 

 part and parcel of the chalk as first deposited and that it 

 subsequently gained its present condition, or whether it 

 was added at certain intervals during the deposition of the 



