620 F. F. GROTJT GRAPHIC STUDY OF IGNEOUS ROCK SERIES 



as ordinates the direct per cent of alkalies. Any criticism of the rise of 

 a norm calculation may, of course, apply to these values, based on the 

 norm. 



Silica. — A close estimate of the excess or deficiency of silica can be 

 made from the mode observed or measured even in the field. The sub- 

 ject is, of course, one where care must be used, and due regard for "satura- 

 tion" and "undersaturation" may be preferred by some petrographers. 11 

 In this country probably more men are familiar with the norm idea than 

 with the idea of saturation. The data for silica are therefore calculated 

 from the norm ; or estimated from the mode by the following plan : 



1. Quartz is estimated directly as excess silica, corrected only for certain 



ferromagnesian minerals as below. 



2. Deficiencies of silica are indicated by the minerals that do not commonly 



occur with quartz, as follows : 



.40 of the olivine present is silica deficiency. 12 

 .73 of the nephelite present is silica deficiency. 

 .27 of the leucite present is silica deficiency. 

 .18 of the analcite present is silica deficiency. 



3. Biotite, hornblende, and augite may occur with quartz or with the minerals 



just listed and modify the estimate, if they are abundant. 

 .26 of the average biotite is silica deficiency. 12 

 .07 of the average hornblende is silica deficiency. 

 .05 of the average augite is silica deficiency. 



It is freely admitted that biotite, hornblende, and augite appear in 

 equilibrium with quartz in some rocks, but biotite does not appear in 

 the norm and may be calculated as deficient in silica, reducing the indi- 

 cated excess in quartz rocks. It is noteworthy that in the field the feld- 

 spars, which dominate most rocks, are the exactly "saturated" minerals 

 with no excess or deficiency, and are at once eliminated from estimation. 

 This greatly simplifies placing the rock in the chart. 



Alkalies. — The sum of soda and potash is plotted directly as ordinates. 

 This may be roughly estimated, from field observations and measure- 

 ments, as the sum of — 



.15 of the potash feldspar, 

 .11 of the sodic feldspar, 



11 S. J. Shand : On saturated and unsaturated igneous rocks. Geol. Mag., vol. 10, 1913, 

 p. 508. 



12 The calculation may be illustrated. In the calculation of a norm, when silica is 

 deficient, the first oxides to be reduced to orthosilicates are the ferrous and magnesian 

 oxides, making olivine. If to 10 parts of average olivine 4 parts by weight of silica were 

 added, it would become hypersthene. In any olivine rock, then, it may be said to be 

 necessary to add silica to the amount of .4 of the weight of olivine before additions de- 

 velop quartz. This .4 of the weight of olivine is the "deficiency" used in plotting. 



