236 JOSEPH P. IDDINGS 



except possibly into those crystallized from magmas rich in soda 

 and alumina. 



The molecule (H,K) Al Si 4 , with little or no iron and with 

 hydrogen and potassium, usually in the proportion of 2:1, occurs 

 as muscovite. It is nearly identical with that occurring in 

 biotite, except for lower iron. Muscovite occurs as a pyroge- 

 netic mineral only in the most highly siliceous and quartzose 

 rocks, where it is sometimes accompanied simply by quartz and 

 alkali feldspar. The fact that this molecule often carries three 

 times as much alumina as potash, and the absence of other 

 potash molecules in which potash occurs in greater amount than 

 alumina, indicates that the muscovite molecule develops in 

 magmas in which alumina is in excess of the alkalis. Its pres- 

 ence bears no direct relation to the percentage of silica in the 

 rock. Its absence from many rocks in which alumina is in 

 excess of alkalis is probably due to its combination with iron 

 and magnesia, resulting in the formation of biotite. Magmas 

 sometimes occur with so large an excess of alumina that it 

 crystallizes as corundum, Al 3 O s , in association with alkali- 

 feldspars. The alumina in these cases did not enter muscovite 

 molecules with the potash. 



The development of the sodium-iron-metasilicate, NaFeSi 2 

 6 , which occurs in pyroxene, as the acmite molecule, and also 

 in riebeckite, is pronounced in magmas rich in sodium and ferric 

 iron ; Fe 2 O s appearing to take the place of A1 2 3 . Considered 

 in connection with the composition of rocks of case I and III, 

 Diagrams 1 and 2, its presence reduces the amount of albite, and 

 increases the silica available for quartz or for raising nephelite 

 molecules to albite according to the position of the rock in the 

 scheme. Its presence would therefore shift the quartz limit 

 toward lower silica percentages. It does not appear to play 

 any considerable role in rocks low in soda or in ferric oxide. 



It is not the plan of the present paper to discuss the effect 

 on the mineral composition of rocks of the development of the 

 less abundant or subordinate minerals. In some cases their 

 production is clearly due to the presence of special elements, 



