618 CROSS, IDDINGS, P IRS SON, WASHINGTON 



for the reason that it is more clearly a function of the composi- 

 tion of the magma, since it depends upon the shapes of the 

 crystals which are often characteristic of particular minerals 

 which preponderate in rocks of certain compositions. 



On the other hand it is possible for any magma to attain any 

 degree of crystallinity, although all degrees, from glassy to 

 holocrystalline, are not known for all kinds of magmas. In the 

 same way any magma may develop any grade of granularity, 

 although some grades are oftener observed in certain kinds of 

 magmas than in others. 



The method of recognizing these differences of texture and of 

 introducing them into the classification is referred to the nomen- 

 clature, where it will be fully stated. 



PART II. NOMENCLATURE. 



Having outlined the system of classification proposed by us, 

 it is necessary'to present and discuss the nomenclature which we 

 have constructed for its expression and use. 



As already pointed out, the present system of nomenclature 

 is most unsatisfactory, for the reasons that it furnishes no dis- 

 tinctions between names of different values (one termination 

 alone being regularly used), nor indication of the relations of 

 any of the groups. 



Furthermore, since the system of classification here proposed 

 is based on relations and principles quite distinct from those in 

 present use, the old names cannot be employed for the new 

 divisions. This is obvious for the reason that the new divisions 

 do not cover the same concepts or characters as the old ones, 

 and because the use of an old name with a new meaning is to be 

 shunned as leading to grave misunderstanding and confusion. 



Rock names, like those of any other system of nomenclature, 

 are, or should be, composed of two parts, with distinct functions, 

 viz., the body or root, and the termination. The former expresses 

 either directly, or by implication or connotation, the character of 

 the object or group of objects to which it is applied, and the 

 latter indicates the place of the object or group in the co-ordi- 



