64 Prof. Grenville Cole — Rhythmic Deposition of Flint. 



Pig. Plate V. 



1. — Spirifer cf. bisulcatus, J. Sow. Ventral valve. * nat. size. Maitai 



Limestone, Martin's Saw-mill. Wairoa Gorge. New Zealand Geol. 



Surv. collection. 

 '2. — Ditto. Internal cast of ventral valve in decalcified rock. Nat. size, 



showing the thickened interior of the ventral valve and dental plates. 



Same locality. New Zealand Geol. Surv. collection. 

 ;'». — Martinia (Martiniopsis ?) subradiata, G. Sow. Ventral valve. Nat. 



size. Maitai Limestone, junction of Wairoa and Boding Eivers, 



Wairoa Gorge. Author's collection. 

 4. — Strophalosia sp. Interior of dorsal and beak of ventral valve. Nat. size. 



Maitai Limestone, Martin's Saw-mill, Wairoa Gorge. New Zealand 



Geol. Surv. collection. 

 5. — Strophalosia sp., cf. Gerardi, King. Gutta-percha squeeze of exterior of 



dorsal valve, ventral beak, area, etc. Nat. size. Same locality, in 



decalcified rock. New Zealand Geol. Surv. collection. 

 <j 8. — Bhynchonella {pugnax) cf. pleurodon, Phill. Gutta-percba squeezes 



of parts of dorsal and ventral valves, area, etc. Nat. size. From 



decalcified limestone at Martin's Saw-mill. Wairoa Gorge. 

 9. -Platyschisma sp. , cf. rotunda, Eth. Nat. size. Maitai Limestone, 



junction of Wairoa and Eoding Eivers, Wairoa Gorge. Dr. Thomson's 



collection. 

 10. — Pleurotomaria (?) or Mourlonia sp. x 4. 



\~la.b. — Serpulites (?) sp. Maitai Limestone, a, section; b. side-view of 

 tube, x 3. 



III. — Thk Rhythmic Deposition ok Flint. 



By Professor GKENVILLE A. J. COLE, M.E.I. A., P.G.S., Eoyal College 

 of Science, Dublin. 



MR. G. W. BULMAN, in an essay on " Chalk Flints aud the Age 

 of the Earth", : states that Professor Owen regarded the layers 

 of flint in chalk as " the remains of successive crops of sponges which 

 grew again and again according to some periodic lata ". Mr. Bulman 

 then observes that the only periods affecting rock-formation are 

 annual, and he suggests that sponge-growth might be rapid during 

 the summer months and that free-swimming reproductive spores 

 might be liberated towards winter, the old sponges then dying off. 

 This implies that the water at the depths in which the sponges lived 

 in our Cretaceous seas was responsive to climatic change, and that 

 the sponges were equally responsive. The suggestion breaks down, 

 however, on another ground, when its author points out that it 

 demands the deposition of 3 or 4 feet of chalk in a year, so that the 

 whole of the English strata above the Albian might have accumulated 

 in about four centuries. Mr. Bulman thereupon remains hesitant and 

 enquiring. 



In the extensive literature on flint-formation, there are probably 

 many suggestions to account for rhythmic deposition. The matter 

 appears, however, to be generally passed over in our textbooks. 

 Sir C. Lyell's reference 2 to changes in marine currents, "favouring 

 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 pre- 

 ponderance of Globigerinse, and in the other of Diatomaceae," seems 



1 Science Progress, No. 41, p. 154, Julv, 1916. 



2 The Students' Elements of Geology, p. 265 : 1871. 



