23 



the original cavity, but apjiaavs to have flowocl out and hardened 

 as molten lead from the opening of a bidlot mould. Flints are 

 found with cavities partially filled with quartz crystals, chalce- 

 dony of mamrailated structure or both combined, when the 

 transition from opaque flint to chalcedony and -clear crystal can 

 be traced. Not only was the presence of sponges inadequate to 

 account for these conditions of flint, but it must be borne in 

 mind that a great portion of the chalk is entirely destitute of 

 flint, though the remains of sponges and organic bodies were 

 abundantly found in it. 



What then is flint ? It is nothing more than altered chalh, or 

 chalk in which the carbonate of lime is replaced by Silica. It 

 will be necessary to go to tlie Chemist, not only to show this 

 is probable, but tliat it has been demonstrated. This process of 

 silicificatiou is not that instantaneous process it is imagined by 

 some, but for the most part a slow pi-ocess, one, moreover, which 

 is going on at present. The chemical histor}' of Silica was then 

 given in more detail than can be conveniently rejiroduced here. 

 Flint consists of nearly piu'e Silica, a combination of Silicon and 

 Oxygen in proportion of one to two. Silica exists in three dis- 

 tinct states, as amorplu'us, graphitic, and crj'stalline. Silica is 

 nearly isomorphic with carbon and its crystals are octahedral 

 On the same sj'stem as the Diamond. Quartz, Chalcedony, 

 Hornestone, and Flint, are all closely allied as varieties of Silica, 

 and possess the same specific gravity, 2-6, are insoluble in boiling 

 alkaline solution, and polarize light. Amorphous flint, however, 

 having a special gravity not exceeding 2-3, does not polorizo 

 light, and is soluble in hot alkaline solutions. The solubility of 

 Silica is of the highest interest to the Geologist. Silica dissolves 

 to a certain e.x.tent in water containing alkaline carbonates, and 

 in such solution fifteen times more amorphous than crystalline 

 silica is taken up. It dissolves in pure water in one pai-t to 

 769-230 according to Bischoff, and is largely present in the hot 

 springs of Geyser and other volcanic waters. A j'^et more im- 

 poi"tant form of Silica is the albuminoid colloid, or hydrated form 

 which it takes on separation from solution under certain terms. 

 One of these methods, in which the gelatmous silica was seen 

 forming di-op by di'op, was most successftdly demonstrated to 

 the audience by Mr. Sidney Harvoj-. Its formation by dialysis, 

 in which the various reagents acted gradually tlu'ough a porous 

 membrane, had a distinct relation to the present consideration, 

 where analogous circumstances were to be found in alternations 

 of porous and non-porous beds, through which mineral solutions 

 might percolate or be ai-rested, separating and depositing their 

 constituents. This liquid form of silica possesses some remark- 

 able properties. It may contain as much as 14 per cent, of silica, 



