Sollas — A Contribution to the History of Flints. 5 



a goodly proportion may quite fairly be regarded as having been 

 cast out from the living sponges. 



It is obvious that experiments are required to determine the 

 rate of production of silica by silicious sponges. Some Greodine 

 sponge, with hispidating spicules (if it could be induced to grow in 

 a tank), would be admirably adapted for investigation : it should 

 be kept in a basin which would admit of clearing out at intervals, 

 so that the quantity of cast-out spicules could be obtained and 

 weighed. 



Although it is clear that the material of flint owes its 

 origin chiefly to silicious sponges, it does not follow that other 

 organisms have not assisted. Schulze notices the comparatively 

 rich development of Hexactinellid sponges in association with 

 diatomaceous ooze, and I have observed something similar in the 

 case of the Tetractinellida. If, then, the contributions of these 

 organisms and of Kadiolaria be taken into account, the time re- 

 quired for the formation of a bed of flints may be still further 

 shortened. 



So far as the evidence goes, it tends to show that a bed of 

 average-sized flints may be formed in a period of about fifty 

 years, more or less — a much shorter period than would, I fancy, 

 have been expected ; and though this case is a single and special 

 one, it possesses also a special value, since it tends, along with the 

 arguments of Wallace, to bring the estimates of geologic time, 

 independently calculated by the physicist and the geologist, into 

 harmony. 



