§6. ORGANIC REMAINS IN FLINT. 301 



which swarm in some of the cretaceous strata. For 

 though silex or the earth of flint is insoluble in boiling 

 water, its solution, as we have previously stated (p. 93), 

 readily takes place in vapour heated to a temperature 

 above that of fused cast-iron (p. 100) ; and similar effects 

 are being produced at the present moment by the thermal 

 waters of active volcanoes (p. 98). 



The temperature to which water may be raised, depends 

 on the presence or absence of air ; for it has been proved 

 by direct experiment that water deprived of air may be 

 heated to 275° of Fahrenheit, under the ordinary pressure 

 of the atmosphere, without exhibiting any symptoms of 

 ebullition. The solvent power of water on rocks containing 

 silex may, therefore, be fully adequate to produce all the 

 phenomena presented by the siliceous nodules, veins, dikes, 

 &c. of the chalk formation ; and our chalk flints may 

 possibly have originated from the quartz of granitic and 

 other plutonic rocks, dissolved in heated water, and erupted 

 into the basin of the ancient chalk ocean. 



6. Organic remains in flint. — In many instances the 

 organic remains in chalk flints are simply incrusted by silex : 

 such is the state of numerous sponges which are as it were 

 invested by the flint, and have their pores and tubes filled 

 by the same material ; the original tissue appearing as a 

 brown earth. In other examples the sponge has been 

 enveloped by a mass of liquid silex, and subsequently 

 perished ; in this manner have been formed those hollow 

 nodules, which on being broken present a cavity containing 

 only a little white powder, or some fragments of silicified 

 sponge : while in other instances the cavity is lined with 

 quartz or chalcedony. It frequently happens that only part 

 of the zoophyte is permeated by silex, while the other 

 portion is in the state of a friable calcareous earth im- 

 bedded in the chalk. 



Sponges and other organic bodies often form the nuclei 



