FA YA 1 TE-IJOLI TE SERIES OF MA GNE T COVE 6 1 5 



much lower, and plagioclase entering to a very considerable 

 extent. 



For this leucocratic holocrystalline combination of ortho- 

 clase (alkali-feldspar) and less nephelite, with hornblende and 

 aegirite-augite, of granitic structure, and with a composition 

 like that given in the analysis above, I would propose the name 

 of Covite. If only the qualitative, not the quantitative, miner- 

 alogical composition be considered, the covites may be called 

 basic nephelite-syenites or foyaites. But the whole tendency 

 of modern petrography is, rightly, against this narrow view of 

 rock classification, and the use of a new name seems to be 

 abundantly justified. In ordinary typical foyaites the alkali- 

 feldspars and nephelite, etc., make up from 75 to 90 per cent, 

 of the reck, the dark minerals consequently only from 10 to 

 25 per cent. In the covites, on the other hand, while the type 

 is rather leucocratic, the light and dark minerals are. present 

 more nearly in the same amount, and these rocks might justly 

 be called " mesocratic." 



As a matter of fact, accepting Pirsson's definition of shonki- 

 nite as the standard (viz., meianocratic combination of alkali- 

 feldspar with pyroxene, etc.), the covites are the rocks which 

 correspond to Rosenbusch's definition of shonkinite. A simi- 

 lar rock, which also belongs here, is that the analysis of 

 which is given in III, and which Cross provisionally called a 

 " tinguaite." 



Arkite {^' Leucite-porpJiyry'). — The specimen which was selected 

 for analysis came from an exposure a little to the northeast of 

 and above Diamond Jo quarry. Judging from the other speci- 

 mens which I collected around the area, it seemed to be repre- 

 sentative and an average specimen of the occurrences. A good 

 sized hand specimen was used for the analysis, so as to obtain a 

 fair sample of this rather coarsely porphyritic rock. 



The results, given in I, were rather surprising in comparison 

 with the analysis by W. A. Noyes of another specimen from the 

 neighborhood (II). Not only is SiOg much lower, but MgO is 

 a little higher, CaO much more so, and, though the total amount 



