624 CROSS, IDDINGS, PIRSSON, WASHINGTON 



A list of all new words proposed by us, with their definitions, 

 will be found in a Glossary which will appear with a reprint of 

 this essay, to be published in book form by the University of 

 Chicago Press. The Glossary and the Tables to aid the calcula- 

 tion of norms are too voluminous to be printed in this Journal. 



ROCK NAMES. 



As just explained, the names to be applied to the systematic 

 divisions, or magma units, are formed from names of geographi- 

 cal localities modified by a series of terminations, and designate 

 rocks so far as their chemical composition and standard mineral 

 composition are concerned, but do not indicate their actual min- 

 eral composition or their texture. The methods of expressing 

 these latter characters in the systematic nomenclature remain to 

 be stated. As already said, each is treated independently of 

 the other. We take up first the expression of the actual mineral 

 composition. 



Actual mineral composition. — A nomenclature that will 

 express the mode of a rock must indicate the kinds of minerals 

 present and their quantity. The question then arises : How far 

 is it necessary to modify, or qualify, the magmatic names in 

 order to accomplish this, and further : What exactness is desir- 

 able in expressing the quantity of the actual minerals present? 



When the name of the magma unit to which a rock belongs 

 is given, there is implied in it the standard mineral composition, 

 or ?iorm, and in order to describe the actual mineral composi- 

 tion, or mode, of the rock it is only necessary to state the extent 

 to which this differs from the norm. 



And since quantitative distinctions within a petrographical 

 unit of the smallest magmatic division established by this system 

 would further subdivide these units, it does not appear desirable 

 in the present state of petrographical science to carry such sub- 

 division to any considerable extent, however desirable it may 

 become in the future. 



If the mode be ?wrmative, that is, if there be complete or 

 almost complete accord between the standard mineral composi- 



