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



largely of liydrated aluminum silicates, such as kaolin 

 and halloysite, also nontronite, iron hydroxides, silicic 

 acid, organic matter, and possibly aluminum hydroxide. 

 These in their finely divided state are all colloidal, and 

 as binders they are stronger when dry than Portland 

 cement. They are the cement of shales and the strength 

 of some limestones. Colloidal silica also serves as a 

 binder in many sandstones, It is known too that the sil- 

 ica cements now in the crystalline state were deposited 

 in some cases in chalcedonic or colloidal condition. 



In addition to this colloidal material in the shales it 

 is known that opaline silica when subjected to the proc- 

 esses of metamorphism loses its H 2 and becomes crystal- 

 line quartz in a mica schist. Clarke 13 shows that colloidal 

 matter contained in most muds and clays is perfectly 

 capable of binding under the influence of pressure alone, 

 and believes that shales owe most of their coherence to 

 unions of this sort. He states on page 609 that it is pos- 

 sible for colloidal silica and colloidal hydroxides of alu- 

 minum and iron to react and form new silicates when the 

 shales are metamorphosed into schists. Thus these finely 

 divided materials play their parts in the make-up of 

 metamorphic products derived from the shales. 



This process must be closely akin to the one going on 

 in the process of binding the materials of our stone roads. 

 Grout 14 and Cushman 15 have been carrying on experi- 

 ments on the cements of roads, and they think their work 

 shows rather conclusively that the binding power of road- 

 making materials is due to substances in the colloidal 

 state, developed by hydrolyzing the oxides, probably of 

 calcium, iron, and aluminum, thus making the body more 

 or less plastic. Subsequently by crystallization and 

 dehydration they become firm, making a solid road. 



Adsorption and Mineral Colors. — Very much can be 

 said on the subject of adsorption and colloids which is 

 not geologic, but a number of points certainly belong in 

 this paper at this place. Every surface has an attraction 

 for other substances. This holding-to a surface is called 

 adsorption. Glue on a board or in a beaker adsorbs 

 board or glass, that is, there is a strong attraction 



13 Clarke, F. W., Data of Geochemistry, U. S. G. S. Bull. 616, p. 545. 



14 Grout, F. E., Journ. Am. Chem. Soc, 27, 1037, 1905. 



15 Cushman, A. S., U. S. Dep. Agr., Bur. Chem., Bull. 85, p. 92 j Trans. 

 Am. Ceramic Soc, 6, 7, 1904. 



