48 ALBERT JOHANNSEN 



A certain system is used for the prefixes. The terms "granite," 

 "syenite," "monzonite," "diorite," etc., are defined, and the 

 addition of a prefix to any one indicates a definite modification. 

 Where no specific name is available, "leuco-" is used to indicate 

 rocks of Class i, "meso-" those of Class 2, and "mela-" those of 

 Class 3. In most cases the prefix "meso-" is unnecessary, since 

 normal rocks belong to Class 2, and these are written without the 

 prefix, the class being understood. Thus there are leuco-granites, 

 granites, and mela-granites, respectively, in Classes i, 2, and 3. 

 Furthermore, syenites, monzonites, granodiorites, diorites, and 

 even gabbros normally contain more than 5 and less than 50 per 

 cent of dark constituents, whereby the prefix "meso-" is unneces- 

 sary. 



Analogous rocks in the four orders of each class similarly have 

 distinctive prefixes where no other names are available. The 

 rocks of Order i have albite as their plagioclase; therefore an albite- 

 monzonite is a monzonite whose plagioclase is albite, and in Order 4 

 an anorthite-monzonite is one containing orthoclase and anorthite. 

 An albite-diorite means a rock all of whose plagioclase is albite; an 

 albite-granite, on the other hand, means a granite containing some 

 albite in addition to orthoclase, since granite itself is defined as a 

 rock consisting of quartz, a biopyribole, orthoclase, and less plagio- 

 clase. That is to say, the term "granite " in itself conveys the idea 

 of an orthoclase rock with some plagioclase, the latter indicated, 

 except in normal rocks, by the prefix. The plagioclase in Order 2 

 is oligoclase to andesine, and that of Order 3 labradorite to bytown- 

 ites. Acid and basic cannot be used as prefixes for these orders, 

 since albite and anorthite, the end members of the acid and basic 

 plagioclases, are set apart as Orders i and 2. Lime-soda and soda- 

 lime are so much alike that one must always stop to think which is 

 which. The prefixes "sodi-" and "calci-" are here suggested. 

 As in the names of normal classes, here also normal rocks drop the 

 prefix; "sodi-," therefore, is seldom necessary. To the rocks of 

 the hinge families, namely those which contain no plagioclase, 

 "ortho-" is prefixed; the feldspar present is orthoclase, microcline, 

 microperthite, or anorthoclase. 



