G. D. Hubbard — Colloids in Geologic Problems. 97 



contact so slowly. Bechhold in 1905, Ziegler in 1906, and 

 Hatschek in 1911 continued experiments with rings 

 in gels. 



Mineral Genesis through Colloidal State. — Liesegang 

 suggested after his studies on the rings in gels that the 

 banding of agates might be due, in many cases, to the 

 slow diffusion of iron salts through silicic acid gels which 

 had previously been laid in cavities or crevices. He also 

 suggested that this process might explain the formation 

 of large crystals in or on quartz in places where igneous 

 activity had not operated, and where they therefore could 

 not be due to its heat and gases. 



In the discussion at the Academy meeting, two points 

 were made. Prof. Gr. F. Lamb reported orally the finding 

 of good agates in recent clays ; the data seem to be want- 

 ing as to whether they developed in preexisting cavities 

 or made their own spaces. Agates in the major cavities 

 of buried bones testify to the recency also of agate- 

 making, and as well to the rate of making. Long ages 

 are not necessary. No evidence was presented as to 

 whether or not the agates came through the gel stage, or 

 how the banding originated. 



As early as 1884, Clarke tells us, Bourgeois 5 made opal 

 synthetically from gels of Si0 2 . Clarke 6 states also that 

 the gelatinous silica formed by the solution of silicates 

 in mineral acids becomes, upon drying, an amorphous 

 mass essentially identical with opal. He also reports 

 that Schafhautl heated a solution of colloidal silica in a 

 Papin digester and obtained a crystalline deposit of 

 quartz ; while de Senarmont heated gelatinous silica with 

 water and carbonic acid gas to temperatures between 200° 

 and 300 C and obtained a crystalline quartz. Chrust- 

 schoff, 1873, obtained quartz from an aqueous solution of 

 colloidal silica by heating to 250° C. for several months. 

 The heat apparently hastened the process, which would 

 have reached the same end in a longer time without so 

 much heat. Silica is dissolved when silicates decompose, 

 and is redeposited by evaporation as opal if still in an 

 acid solution, but when alkalies are present it crystallizes. 



When the Simplon tunnel in central Switzerland was 

 put through the Alps, the engineers found a vein of silica 



'° Bourgeois, L., Production of Minerals Artificially, p. 93. French. 

 G Clarke, F. W., Data of Geochemistry, U. S. G. S. Bull. 616, p. 357. 



