H7 



siliceous material would separate from the limy mud by a process which 

 ;,^oes on in the manufacture of pottery. When the ground flints have 

 been reduced to a fine powder, and then mixed with clay in the soft 

 putty-like condition, there is a tendency in the silica to separate from 

 the rest and run together into nodules. It was necessary to prevent 

 this by constant agitation." 



Here, at anyrate, might be a possible solution of the growth of a 

 flint nodule. No one could examine a collection of flints without being 

 struck by the fact that organic matter had been a most active agent in 

 causing the precipitation of silica from its solution and so of determining 

 the forms of flints. They were well aware what a large part sponges 

 had borne in their formation. Sponges were generally developed in a 

 gelatinous matter, and it was an interesting, fact that in an artificially 

 prepared solution of silica, -one of gelatine immediately brought about 

 a coagulation of it. Organic substances in their decay evolved 

 carbonic acid. A few bubbles of this gas passed through a solution 

 slowly brought about a coagulation. There is no body with which 

 silica seemed to have so much affinity as a compound of iron. 

 The red stains in flint bore v,-itness to it. A solution of ferric oxide also 

 rapidly precipitated the silica firom its solution. Had time not been so 

 short he should have had great pleasure in showing them some of 

 these experiments. To the power which silica possessed of replacing 

 carbonate of lime, and on which Professor Rupert Jones laid so much 

 stress in the formation of flints, he could not do much more than 

 briefly allude. The most interesting problems in the study of these 

 bodies were to be found connected with hollow flints. Here the silica 

 had been deposited round a more or less perishable or soluble substance, 

 which, subsequently or contemporaneously with the deposition of the 

 silica, had been removed. But there was one general law connected with 

 them, viz.,— that whenever the purer ciystalline or semi-crystalline 

 varieties of silica occurred in connection with flint, they were always 

 found in the iiiHrior and never on the exterior of it. 



Chalcedony differed from flint but slightly in its chemical consti- 

 tution, but largely in its structure. It was mammillary, botryoidal, 

 stalactic, with radiating fibrous lines at right angles to the plane of 

 deposition. It generally occurred lining or filling cavities in rocks. 

 Varieties of it were Beekite, Carnelian, Chrysoprase, Onyx, Agate, &c. 

 Specimens of these were before them, mostly from his own collection. 



