604 CROSS, IDDINGS, PIRSSON, WASHINGTON 



cases in which more or less of the magma becomes glass will be 

 discussed afterward. 



For convenience we introduce two terms which have sug- 

 gested themselves as substitutes for the cumbrous and oft- 

 repeated expressions, standard mineral composition (that calcu- 

 lated from the rock analysis) and actual mi7ieral composition. For 

 the first we propose the word norm' 1 , and for the second the word 

 mode. 2 The standard minerals which make up the norm are to be 

 called the ?iormative 3 minerals, not the normal ones, since the lat- 

 ter adjective has the meaning of usual or common. 



The questions arise, how far does the mode of a noncrystal- 

 line rock correspond to the norm, and what distinctions are 

 necessary among rocks belonging to one magma unit because of 

 differences among them due to their actual mineral composition ? 



There are all possible grades of correspondence between the 

 mode and norm of rocks when the whole range of rocks is 

 taken into account, but when this inquiry is applied to separate 

 magmatic units the answer must be modified according to the 

 kind of magma considered. 



For example, some magmas of extremely simple composition 

 crystallize in one manner only, so far as our present knowledge 

 goes. Those composed of almost pure silica form only quartz 

 with negligible amounts of other things. Magmas whose norm 

 is quartz and orthoclase will form rocks with these minerals as 

 the mode. Magmas whose norm is a simple rock-making min- 

 eral, as labradorite or olivine, will crystallize into rocks composed 

 of one of these minerals only. 



Consequently rocks belonging to magmatic divisions char- 

 acterized by extreme amounts of standard quartz, orthoclase, 

 albite or nephelite ; of the lime-soda-feldspars; of diopside or 

 hypersthene, or olivine, or magnetite, will have actual mineral 

 compositions very closely corresponding to the calculated norm. 

 That is, the preponderant minerals actually crystallized will be 



1 A rule, pattern, model, authoritative standard. — Century Dictionary. 



2 The natural disposition or manner of existence of anything. — Ibid. 



3 Establishing or setting up a norm. — Ibid. 



