ROCK CLASSIFICATION ON THREE CO-ORDINATES 211 
and (often) ferric iron. Mediosiliceous alkaline rocks contain calcic 
plagioclase and a little lenad or alkali feldspar, and micas or soda- 
amphiboles or soda-pyroxenes; or they contain acid soda-lime 
feldspar with equal alkali feldspar and with micas, pyroxenes, 
amphiboles; chemically they contain in the first case high alkalies, 
lime, and titanic acid and low magnesia and silica, and in the second 
case high alkalies with potassa equal or dominant over soda, and 
abundant lime and alumina. Persiliceous alkaline rocks are 
characterized by dominant alkali feldspars, no soda-lime feldspar 
nor lenad, and lithia-mica, soda-amphibole, soda-pyroxene; they 
are very rich in alkalies and rarely in ferric iron, and very poor in 
lime and magnesia, and contain insufficient alumina to saturate the 
alkalies. 
In general, alkaline igneous rocks contain sodic mafic silicates 
and no feldspar, or they contain more alkali feldspar and less soda- 
lime feldspar than the corresponding alkalcic rocks; chemically 
they contain more alkalies and less lime than the latter. 
Peralkaline rocks are characterized mineralogically by the 
presence of feldspathoids (or lenads); they commonly contain also 
soda-pyroxene or soda-amphibole, or both. Chemically they are 
distinguished by insufficient silica to combine with the abundant 
alkalies to form feldspars after saturation of other available bases 
as orthosilicates. 
In addition to the alkalcic, the alkaline, and the peralkaline, 
other divisions might be recognized, for example, the superalkaline, 
in which feldspars are entirely replaced by lenads. But this 
division consists of rocks which are very rare, and therefore it may 
be conveniently regarded as merely an extreme variation of peral- 
kaline rocks. In spite of their rarity many names have been pro- 
posed for superalkaline rocks. Thus, by loss of feldspar, nepheline 
syenite becomes urtite, nepheline porphyry becomes sussexite, 
phonolite becomes leucitophyre, theralite changes to fergusite, 
tephrite to nephelinite or leucitite, olivine theralite to missourite, 
basanite to nepheline basalt or leucite basalt, and the volcanic 
equivalent of teschenite to analcite basalt. Another division might 
consist of the subsiliceous or lamprophyric rocks; but, with rare 
exceptions, such as shonkinite, the lamprophyric rocks are related 
