1849.] BOWERBANK ON A SILICEOUS ZOOPHYTE. 323 



the eye, as if the blade of a sword were deliberately passed through 

 the solution, and this under the influence of the polarizing apparatus 

 exhibits perhaps a uniform green colour according to the thickness 

 of the crystal ; after this there is a cessation of action for a period, 

 and then another layer of the substance of the salt passes over the 

 surface of the crystal from its base to its summit, and it then becomes 

 as universally and vividly pink as the first layer was green, and thus 

 layer after layer is added and the colours continue to alternate until 

 the action ceases. 



Within the boundaries of the fossilized body of the Alcyonium 

 there are no appearances of crystalline arrangement, but it is im- 

 mediately on, or slightly without the outer surface, that the first 

 crop of crystals is based, and the succeeding one follows from the 

 apices of the first, and so on, crop succeeding crop, until the inter- 

 vening spaces are entirely filled up. Sometimes the force of the 

 projected crystals has been sufficient to thrust before them a con- 

 siderable quantity of the substance of the organic matter in course of 

 fossilization, which is finally crushed into a dense mass between the 

 opposed crops of crystals, when their apices meet. Such is frequently 

 seen to be the case in masses of fossilized wood, both siliceous and 

 calcareous, and the force exerted by crystallization is evidentl}^ for- 

 ward, and in no case that I have seen is it backward ; for the tissues 

 at the bases of the respective crops of crystals show not the slightest 

 evidences of compression, while the cellular and vascular structures, 

 at the junction of the opposed bodies of crystals, exhibit every 

 evidence of having been forcibly driven forward, and finally com- 

 pressed into a solid opaque mass, affording only sufficient evidence 

 of structure to be certain of the nature of the material. 



These phgenomena, it will be observed, accord with the mode of 

 the production of prismatic crystallization which I have described as 

 occurring beneath the microscope ; thus rendering it highly probable 

 that the calcedonic crystallization of silex is an operation achieved in 

 very much less time than may have been imagined. 



After a period, the tendency to produce crops of calcedonic crystals 

 appears to cease, and hollow spaces remain, which are completely 

 bounded by the apices of the last crop of crystals ; and these spaces, 

 and the cessation of prismatic crystallization, are probably produced 

 by the complete exclusion of further portions of silex in solution : 

 but at other times, in lieu of the spaces, we find a solid mass of 

 regular crystals of quartz ; and in this case it is probable that the 

 surrounding fluid, holding silex in solution, had access to the cavity, 

 formed by the apices of the last crop of calcedonic crystals, through 

 some minute opening, so that a contiimous but slow change of the 

 fluid took place in accordance with the laws of endosmose, as soon as 

 the silex was deposited within. 



These views, regarding the deposit of silex in solution, are to a con- 

 siderable extent confirmed by an interesting fact communicated to me 

 by my friend Mr. Warren De la Rue, who found minute crystals of 

 silex deposited on the inner surface of a phial, which contained the 

 residuum of an analysis in which there remained some of that earth 

 in solution. 



