Nodules of the Trimmingham Chalk. 445 



time. For instance, the silica of tlie skeletons occurs in con- 

 junction or probably in combination with an organic basis 

 known as spiculin • on solution it is liberated from the spi- 

 culin and exists in the colloid state, whence it readily passes 

 into the pectous condition, and subsequently becomes crystal- 

 line ; it is, moreover, probable that, under conditions not yet 

 investigated, a solution of colloid silica may give rise directly 

 to silica in a crystalline form. If it be objected that in this 

 expanded explanation, fact and conjecture are mixed together, 

 I to some extent admit it, but at the same time remark that 

 there is no conjecture in the statement that the silica which 

 passes into solution is a very different thing from the silica 

 which has passed out of solution. The one may be con- 

 veniently called organic, and the other mineral silica ; the 

 properties of the two are strikingly different ; and the process 

 which has really happened has been a solution of organic 

 silica and a deposition of mineral silica, not a solution and 

 deposition of the same kind of silica. In the next place, solu- 

 tion and deposition need not have proceeded pari passu ; 

 if one succeeded the other only after a considerable inter- 

 val, there would be time for the conditions to change : 

 an elevation of the sea-floor and a consequent shallowing 

 of the sea might, for instance, have intervened ; and if we 

 suppose the silica to be held in solution through the influ- 

 ence of hydrostatic pressure, the diminution of this pressure 

 would lead to its deposition. There are difficulties, however, 

 in the way of this supposition which lead me not to lay great 

 stress upon it. 



We shall now proceed to consider the different modes in 

 which the deposition of silica has been effected. Of these 

 there appear to be three, viz. the simple deposition of silica, 

 its deposition as a pseudomorph after carbonate of lime, and 

 in combination with bases forming silicates. 



(1) Simple deposition of the dissolved Silica. — (a) The 

 simplest case of this is presented by the minute crystals of 

 quartz which frequently occur dispersed through the substance 

 of limestone beds ; a figure of these is given in my paper on 

 Catagma (loc. cit. p. 361). They are mentioned by Zittel 

 (Lehrbuch der Petrologie, 1866) and fully described by Mr. 

 T. Wardle of Leek, in his presidential address to the North- 

 Staffordshire Field-Club in 1873. As the 'Proceedings' of 

 this society may not be generally accessible, I venture to 

 quote Mr. Wardle's description in full. He says : — 



" My friend Mr. Woodcraft, who has made a careful exa- 

 mination of the Mountain Limestone of Caldon Low, gives 

 the following as the result of his analysis : — carbonate of 



