162 THE FORMATION OF FLINTS 



growth, death, and decay of a host of calcareous organisms, 

 and continuously in process of formation ; here and there 

 beds of various kinds of sponges flourished in rich pro- 

 fusion, twenty or thirty species, or even more, might 

 have been collected at a single sweep of the dredge, just 

 as happened with some of the most successful hauls 

 of the Challenger in the existing ocean, These sponges 

 built up siliceous skeletons formed of opaline network or 

 variously-shaped needles ; the latter they had a habit of 

 shedding during their lifetime, extruding them from their 

 surface till they fell out and mingled with the ooze of the 

 sea floor. Periodically they produced swarms of young, 

 and at their death left behind a rich legacy of silica. 

 Possibly other siliceous organisms, diatoms, and radio- 

 laria contributed to the increasing accumulation of silica, 

 but direct evidence of this is rare or wanting. Then, as 

 the result of some change of circumstances, the sponge life 

 grew rare, the growth of chalk predominated, till again some 

 alteration occurred and fresh fields of sponges covered the 

 sea floor, to be smothered up once more beneath the all- 

 pervading chalk, and thus by slow mutations a great 

 thickness of chalk and sponge beds accumulated within 

 the ocean basin. Then came a revolution of the times ; 

 the sea gave place and made way for the ascending land ; 

 the sheets of chalk were thrown into mighty rolling folds 

 and rose into the air. Within the mass of the rock a 

 fresh series of events was set in train ; molecules of silica 

 and lime carbonate were set marching and counter- 

 marching this way and that, till at length the silica had 

 gathered itself together in concentrations of flint. Eain 

 and rivers, wearing away the softer chalk, left the hard 

 flints exposed upon the ground or rolled them into 

 pebbles. Then a creature of but yesterday, in whom the 

 life of the earth attains its intellectual culmination, came 

 by and found in these pebbles and nodules a substance 



