272 



PKOCBEDINGS OF SECTION C. 



element which is relatively inactive chemically, being found some- 

 times in the basic, sometimes in the acid radical, is oftenest combined 

 with the strongest base-forming elements, potassium and sodium. 

 These relations are illustrated by the following common, simple, pyro- 

 arenetic minerals : — 



It is well known that the orthosilicate of sodium and aluminium 

 (nephelite) and the metasilicate of potassium and aluminium (leucite) 

 do not form in the presence of free silica (quartz), while metasilicates 

 and orthosilicates of the less active metals, calcium, magnesium, and 

 ii'on (pyroxenes, olivine and anorthite) do. The relative chemical 

 activity of all of the elements conmrion to igneous rocks may be 

 illustrated in like manner, and the probabilities of various pyrogenetic 

 m.ineral compounds forming from different rock magmas may be made 

 clear. 



One of the most important factors in the discussion of the 

 chemistry of igneous rocks is the role of hydrogen, whether as an 

 active base-forming element, or as a catalytic agent, alone, as hydro- 

 gen (H), or combined with oxygen, as hydroxyl (OH). Its exact 

 behaviour in each specific case is not definitely known, but the 

 principles applicable to several distinguishable cases are clearly estab- 

 lished. 



Adopting the idea that an acid is a hydrogen salt in which 

 hydrogen plays the role of a positive, base-forming element, an acid 

 salt may be considered as one in which all of the hydrogen has not 

 been replaced by other base-foiTiiing metals. Such an acid salt may 

 be looked upon as a substitution derivative from a hydrogen salt 

 (acid), or from a normal salt by the introduction of hydrogen in place 

 of other positive metals. An example of such a compound among 

 pyrogenetic minerals is to be found in muscovite (K,H)Al(Si04). 

 This might be derived from H4Si04, KAl(Si04) or Al4(Si04)3. The 

 formation of such a compound involves the presence of active hydrogen 

 to play the role of metal. Muscovite is a common pyrogenetic mineral 

 in some igneous rocks rich in silicon, with much uncombined silica, 

 and also in others comparatively low in silica, accompanying ortho- 

 silicates and nephelite. It forms by the side of polysilicates — ortho- 

 clase and albite — and even with the disilicate, petalite. The forma- 

 tion of muscovite must be ascribed to the action of hydrogen upon 

 silicon, either directly in the first instance, or, if previously foi'med 

 silicates of aluminium and potassium be assumed to be the source of 



