Nodules of the Trimmingham Chalk. 451 



is true that in most cases silica so deposited takes a crystalline 

 form ; but crystals of silica in the presence of chalk are of 

 rare if not of unknown occurrence. We do certainly find 

 them in the interior of some flints ; but then they are seated on, 

 and surrounded by, the silex, and are nowhere in actual con- 

 tact with the chalk itself. It would indeed appear that the 

 simple deposition of silex is impossible in the chalk; the 

 first stage of deposition in this deposit is always that of re- 

 placement. 



The deposition of the silica being determined as to place 

 by the presence of the fissure, began, as we might expect, 

 by a replacement of the chalk, silicifying the walls of the 

 fissure ; subsequently, as in the Trimmingham flints, a simple 

 deposition of silica followed, cementing the siliceous chalk 

 into compact structureless flint, and a flint vein was the 

 result. 



It will be seen that we merely make use of the fact that 

 free surfaces are often surfaces of deposition, without explain- 

 ing it ; but to enter fully into this subject would be beyond 

 the scope of the present paper. 



3. Flints formed about Sponges. — The irregular nodules may 

 be left to be dealt with after we have considered those forms 

 in which the general outline of some known sponge, such as 

 a Ventriculite or Siphonta is represented, or more generally 

 those which have obviously been formed about some kind of 

 sponge-skeleton. The characters of these are well known. 

 One meets with, say, an egg-shaped mass of flint ; and on 

 breaking it open a conical Ventriculite is seen in the middle ; 

 sometimes the form of the Ventriculite is more closely repre- 

 sented, the vasiform skeleton being merely coated inside and 

 out by a layer of flint, often about an inch thick ; while occa- 

 sionally a Ventriculite may be met with simply girdled by a 

 ring of flint round the middle, the rest of the sponge remain- 

 ing unenveloped. Sometimes the body of the sponge is en- 

 closed, and not the roots, sometimes the roots and not the 

 body. The sponges included are of various kinds — Siphonia, 

 Poterion, and, in one instance known to me, a Tethya. 



In attempting to find an explanation for the form of these 

 flints we may consider the following suppositions : — (i) The 

 form may have been determined by the presence of animal 

 matter (protoplasm, Wallich), or (ii) of the products of 

 its decomposition, or by the presence of the siliceous skeleton 

 of the sponge, either (iii) through a special attraction exerted 

 between it and silica in solution, or (iv) by its furnishing 

 an extra supply of silica to the surrounding water, or (v) 

 on account of its providing a free surface of deposition. 



