156 THE FORMATION OP FLINTS 



presence of this substance as one of its constituents ; but 

 the quantity is amazingly small, 14 parts of silica in 

 1,000,000 of sea-water would perhaps be rather above 

 than below the average. Evidently the sponge would 

 require to be brought into contact with a very consider- 

 able amount of water before it produced a skeleton of any 

 appreciable size. The question seemed to me worthy of 

 a little investigation, which led to the following result : 



FIG. 45. A Cube to represent a Sponge having 

 a volume of 58 cubic centimetres, with a 

 prism indicated by chequers to show the 

 volume of silica it would contain. 



Experiments made on eleven different kinds of sponges 

 showed that the siliceous skeleton formed on an average 

 13 per cent, of the total bulk of the organism, so that a 

 sponge 58 cubic centimetres in volume* (Fig. 45) a fair 

 average size may possess a skeleton containing 14 grams 

 of silica, or as much as is contained in 1,000 litres of sea- 

 water, the weight of which is considerably over a ton. 



* A volume of 60 cubic centimetres would be represented by a 

 rectangular prism 3 c.m. by 4 c.m. in section and 5 c.ni. high. 



