CLA SSIFICA TION OF IGNE OUS RO CKS 6 2 1 



proper selection on mnemonic grounds, give an idea of t h 

 character. 



The latter method would seem at first sight to be the better, 

 since it apparently involves less tax on the memory. We have, 

 however, made many attempts at the construction of such name 

 roots, giving them practical trials at all points of the scheme, but 

 have been forced to reject this method for all divisions except 

 that of Class, for the following reasons : 



1. The names are apt to be excessively long and cumbrous, 

 especially in the smaller divisions. 



2. While the distinctions can be readily seen on paper, yet 

 close attention must be paid to each syllable and even to each 

 letter, and in spoken language the names are so similar as to be 

 confused with each other. 



3. As a consequence, the mnemonic quality, which is the great 

 theoretical advantage of this method, is very seriously dimin- 

 ished, especially in the smaller divisions. 



4. The adoption of this method involves a fixation or lack of 

 elasticity in the nomenclature, which would be fatal to it, if the 

 need arose for any change either in the method of arriving at 

 the divisions or in their number, since this would involve a cor- 

 responding change in the nomenclature. 



We have, therefore, except for Classes and Subclasses, adopted 

 roots derived from names of localities, taking advantage of the 

 fact that there are at present many of these in present petro- 

 graphical nomenclature with connoted magmatic ideas which 

 are readily adaptable to our purpose. Thus with the root 

 nordmark is connoted the idea of an alkali-feldspathic rock, 

 with miask that of a rock composed essentially of alkali- 

 feldspar and nephelite. These are, it is true, not the only con- 

 notations of these names, since in both of them are also 

 implied ideas of texture. 



In the names suggested subsequently we have endeavored to 

 adhere to the following principles in the selection of roots. 



a) They should be adjusted to the relative position of the 

 divisions to which they are applied. That is, the roots of names 



