188 Dr. Wallich on the Origin of the 



change of form. On the contrary, we have in almost every 

 pseudomorph, whether consisting of carbonate of lime after 

 silica, or silica after carbonate of lime, or an admixture of both 

 carbonate of lime and silica, a well-defined retention of the 

 general outline of the object, although extending only to the 

 ghostly remnant of the organic basal matter to which reference 

 is made. 



The necessary evidence is, I submit, therefore complete, of 

 the black flint* not being the product, in any sense, of the 

 replacement of one mineral substance by another, but the 

 direct resultant of the gradual transition of its silica from 

 a gelatinous to the " pectous " condition, during which the 

 last removable vestige of its " hydration" is expelled and the 

 production of " the hard stony mass of vitreous substance " 

 called flint is consummated. 



As these remarks apply more or less to the entire flint- 

 formation, including the cherty varieties, I hereto append a 

 few short passages from Mr. Graham's paper on silicic acid, to 

 which I have already been so deeply indebted for guidance in 

 the present inquiry, as I should of course wish to give the 

 whole weight of that illustrious physicist's scientific authority 

 to the statements that have been put forward on the subject. 

 Having done so, I shall consider my case concluded, so far as 

 the mode of production of the flints is concerned. 



" A dominating quality of colloids," Mr. Graham wrote, 

 " is the tendency of their particles to adhere, aggregate, and 

 contract. This idio-attraction is obvious in the gradual 

 thickening of the liquid, and, when it advances, leads to 

 pectization. In the jelly itself the specific contraction in 

 question, or synceresis, still proceeds, causing separation of 

 water, with the division into a coagulum and serum, and 

 ending in the production of the hard stony mass of vitreous 

 substance, which may be anhydrous, or nearly so, when the 

 water is allowed to escape by evajjoration. . . . Bearing in 

 mind that the colloidal phasis of matter is the result of 

 a peculiar attraction and aggregation of molecules, never 

 entirely absent from matter, but greatly more developed 

 in some substances than in others, it is not surprising that 

 colloidal substances spread on all sides into the liquid and 

 solid conditions. ... It is unnecessary to return here to the 

 ready pectization of liquid silicic acid by alkaline salts, 

 including some of very sparing solubility (such as carbonate 

 of lime) , beyond stating that the presence of carbonate of lime 



* Throughout my paper I have spoken of the black Jlint and the 

 typical Jlint of the upper or ivhite chalk, only because the characters I 

 wash to account for are most strikingly seen in it. 



