482 THE DEPTHS OF THE SEA. [chap. x. 



work of spaces, vacant, or loosely filled with peroxide 

 or carbonate of iron. It therefore seems certain that 

 by some means or other the organic silica, distributed 

 in the shape of sponge spicules and other silicious 

 organisms in the chalk, has been dissolved or reduced 

 to a colloid state, and accumulated in moulds formed 

 by the shells or outer walls of imbedded animals of 

 various classes. How the solution of the silica is 

 effected we do not precisely know. Once reduced 

 to a colloid condition, it is easy enough to imagine 

 that it may be sifted from the water by a process 

 of endosmose, the chalk matrix acting as a porous 

 medium, and accumulated in any convenient cavities. 

 In various localities in the chalk and green-sand of 

 the North of England the peculiar bodies which are 

 called Ventriculites are excessively abundant, — ele- 

 gant vases and cups with branching root-like bases, 

 or groups of regularly or irregularly spreading tubes, 

 delicately fretted on the surface with an impressed 

 network like the finest lace. In the year 1840 the 

 late Mr. Toulmin Smith published the result of 

 many years' careful study of these bodies, and gave 

 a minute and most accurate description of their 

 structure. He found them to consist of tubes of 

 extreme tenuity, delicately meshed, and having be- 

 tween them interspaces usually with very regular 

 cubial or octohedral forms. These tubes in the Ven- 

 triculites found in chalk were empty, or contained a 

 little red ochreous matter ; but when a ventriculite or 

 a portion of one happened to be entangled in a flint, it 

 was either incorporated with the flint or replaced by 

 silica. Mr. Toulmin Smith supposed that the skeleton 

 of the ventriculite had been originally calcareous, 



