FAYAITE-IJOLITE SERIES OF MAGNET COVE 



617 



in II, it is impossible to make a satisfactory calculation of 

 Noyes' analysis, but No. I works out thus, the result being only 

 an approximation, owing to the composition of the rock. In 

 Ilia is given that of the missourite, as calculated by Pirsson. 



It is evident from this table that while both rocks are alike 

 in being composed essentially of leucite, with subordinate 

 nephelite (or zeolites), and dark minerals, yet that they differ 

 radically from each other, just as did the covite and shonkinite. 

 The Magnet Cove rock is distinctly leucocratic, carrying about 66 

 per cent, of light minerals, while the missourite is as decidedly 

 melanocratic, carrying only 24 per cent, of these. 



It is obvious from the mineralogical composition, as well as 

 from the analysis, that the name "syenite" which has been 

 applied to this rock is not justified, if this term is to retain any 

 precision of meaning except that of indicating the absence of 

 quartz and an alkaline character. Since this is so, and since the 

 rock represents a most interesting and quite distinct type, it cer- 

 tainly should have a distinct appellation of its own. 



It would seem peculiarly appropriate to honor the memory 

 of its first describer, J. F. Williams, by calling it Williamsite. 

 But since this name has been already preempted by Shepard for 

 a variety of serpentine, and as it would be a solitary exception 

 among rock names, it will be best not to do so. I propose, 

 therefore, the name of "arkite" (from the usual abbreviation of 

 the state name Arkansas), the essential features being a holo- 

 crystalline, porphyritic, leucocratic combination of leucite (or 

 pseudo-leucite) and nephelite, with pyroxene and garnet. 



