STUDY OF IGNEOUS ROCKS. 271 



of the final products. The chemical behaviour of ionic substances, and 

 especially the hydrolysing action of ionised water are other factors in 

 the problem under consideration. The relative strength of chemical 

 activity in the base-forming, or acid-forming, elements and their 

 ability to form acids and salts, lead to the discussion of the production 

 of the pyrogenetic minerals from magmas composed of elements found 

 in igneous rocks ; many of these minerals having been produced in the 

 laboratory by melting together the component elements in proper 

 proportions. 



Considering what should take place in a solution having the com- 

 position of an average of all igneous rocks, it can be shown, since the 

 chief acid-forming elements present are silicon in large amount, and 

 ■ the more active element phosphorus in yery small amount, that salts 

 with these elements in the acid radical must be common. Other acid- 

 forming elements occurring in small amounts are titanium and zir- 

 conium ; while iron and aluminium may play this role under favour- 

 able conditions. The more active phosphoric acid forms unstable 

 salts with the active base-forming metals, potassium and sodium, but 

 a very stable compound with the less active metal, calcium, into which 

 compound fluorine, or chlorine, enters; yielding apatite, an almost 

 universal component of igneous rocks. 



Silicon is known in the laboratory to form one definite acid, 

 H4SiO,i, orthosilicic acid; and other acids of silicon have not been 

 isolated and identified. But very definite mineral compounds exist 

 which indicate that salts from other silicic acids form under proper 

 conditions. These are: — 



H4Si04 orthosilicic acid. 

 H2Si03 metasilicic acid. 

 £[481308 polysilicic acid. 

 H2Si205 disilicic acid. 



It is significant that in laboratoiy experience with orthosilicic acid, 

 1148104, prepared from aqueous solutions, the compoimd may be made 

 to lose water gradually until nothing but silica, Si02, remains. In this 

 way free silica may be separated from a silicate compound, a hydrogen 

 silicate ; since an acid may be considered as a hydrogen salt. 



Obsei-vations upon the pyrogenetic minerals, and laboratory ex- 

 perience with synthetical operations, show that salts of several kinds 

 of silicic acids form by the side of one another, and that their 

 character and amount depend on the nature of the base-forming 

 elements present in the mixed solution. Orthosilicates, metasilicates, 

 and polysilicates commonly fonii in the presence of one another, some- 

 times accompanied by uncombined silica. And it becomes more and 

 n^ore evident that the formation of the differ^-at kinds of silicic acid 

 ions, or their salts, is controlled primarily by the strength, or chemical 

 activity, of the base-forming elements: is dependent also on the 

 amount of silica available in the solutio*L ; and may be modified, of 

 course, by other factors. Thus, it appears that the most active metals 

 command the highest silicic ions. The highest silicates common in 

 igneous rocks are the polysilicates of the alkalies, potassium and 

 sodium-orthoclase and albite. 



Further, the abundance of aluminium in most rock magmas 

 results in the presence of abundant aluminous compounds. And this 



