18 



hairs in a mortar. It stuck together in lumps as if there were 

 white of egg mixed with it ; this was the sarcode of sponges." 



In the beautiful Hyalonema Sieboldii and Pheronema Grayi 

 we have examples of recent sponges whose beautiful glassy root 

 fibres interpenetrate the floor of the Atlantic as Sir W. Thompson 

 describes. 



Here again is a modern siliceous sponge from Barbadoes, 

 which very few would take to be a sponge at all, so much it 

 resembles a piece of somewhat light and porous sandstone, formed 

 into a vase-like shape. 



PART II. 



PEOBLBMS SUGGESTED BY THE EXAMINATION OF 



INDIVIDUAL SPECIMENS OF FLINT. 



From the foregoing we have obtained some idea of the 

 manner in which, in all probability, the chalk of the ancient seas 

 was deposited, and the circumstances under which silica was 

 associated with it. Let us now see from an examination of some 

 specimens of flint and flinty concretions which are furnished to 

 us by the unique collection which the Brighton Museum owes to 

 the enthusiasm and liberality of Mr. Henry Willett, what further 

 questions they suggest, and what light they throw on the problems 

 presented to us. 



And first of all it will be necessary to define more 

 accui'ately what flint is. Scientifically then flint " is a sub-trans- 

 lucent amorphous variety of chalcedony, having little or no action 

 on polarized light." Its very blackness being, according to some, 

 only an optical effect, as water is itself dark blue or black under 

 certain conditions. Pounded flint is quite white. 



The definition above given demands some explanation. For 

 the further question naturally arises — what is chalcedony ? 

 Briefly then the Protean forms of this interesting substance 

 silica — rock-crystal, quartz, amethyst, cairngorm, agate, 

 chalcedony, cornelian, opal, and many others may be 

 classified under three heads — crystallized silica — such as quartz 

 and rock crystal, crypto-crystalline, such as chalcedony, in which 

 incipient crystallization is shown, and which has some action on 

 polarized light, and the amorphous — formless — colloid silica. 

 The word colloid is derived from a Greek word signify- 

 ing glue or gum. Colloid silica is a gelatinous form, which 

 is largely soluble in akalies and some acids. It enters readily 





