8 N. L. Bowen — Genetic Features of 



cellite present. Except in its unusnally low melilite, 

 specimen II is fairly typical of the average. 



Comparison of the two analyses shows no very marked 

 differences in spite of the considerable contrast in pro- 

 portions of the constituents. Lime is even higher in the 

 variety poor in melilite because it is correspondingly 

 richer in monticellite (and in minor lime-bearing min- 

 erals) and the lime content of monticellites and melilite s 

 is about the same. Magnesia is higher in I on account 

 of the greater amount of chrysolite. The lower biotite 

 content of I shows up in the lower potash and alumina. 

 The biotite must contain mucli soda. The lower silica of 

 II can not be assigned any special significance on account 

 of its greater alteration (note C0 2 ). 



Comparison of the analyses with those of other alnoites 

 shows the present types to be higher in magnesia. Soda 

 in excess of potash as in I is exceptional in alnoites which 

 ar§ usually like II in this respect in spite of their common 

 association with sodic rocks. This is connected with the 

 low biotite content of I; in fact, this type, in virtue of 

 decreased biotite, leans toward melilite basalt. 



Specimen I is apparently the freshest alnoite yet 

 analyzed and even II compares favorably with others in 

 this respect. 



No new names are proposed for these rocks nor for any 

 of the types described in this paper. It is believed that 

 the descriptive terms employed have a distinct advantage 

 over the meaningless names based on locality that are 

 being coined daily and with which the literature is already 

 overburdened. 



Melilite-Biotite Rock. 



Before going on to discuss the relations of one mineral 

 to another in the principal type whose mineralogy was 

 given above, the streaks and patches that have been 

 mentioned as occurring in it will be described. The 

 streaks are from 1 inch to 2 feet wide and traverse the 

 main type after the manner of dikes or veins with 

 indefinite boundaries. They consist almost entirely of 

 biotite and melilite with subordinate amounts of apatite, 

 chrysolite, monticellite, perovshite, and an opaque ore 

 mineral. 



