296 Weed and Pirsson — Bearpaw Mountains, Montana. 



This analysis shows very clearly the alkaline character of 

 the rock and its low lime, iron and magnesia. The chlorine 

 and fluorine are in the small amount of apatite. The sulphur 

 shown must come from the traces of sulphide ores present ; the 

 titanic oxide is probably mostly present in the augite ; a little 

 titanite may be present, but has not been seen. The barium 

 and strontium are probably present in the feldspars. The 

 small amount of water shows the freshness of the rock. Its 

 simple mineral composition permits us to calculate from the 

 analysis the proportion of the different mineral molecules 

 which compose it as follows : 



^girine augite 8*9 



Orthoclase 34-4 



Albite 48*5 



Anorthite ] -7 



Quartz ti'o 



100-0 



The amount of the anorthite molecule is a trine ' too 

 large, the small quantity of lime required to form the apatite 

 not having been deducted. The augite would have the com- 

 position previously given. 



For purposes of comparison some analyses of quartz syenites 

 from other parts of the world have been quoted. (The one 

 quoted from Highwood Peak is from a report -in preparation 

 by the authors on the mountain group of that name.) These 

 are all quartz-alkali syenitic rocks and they belong to Brogger's 

 Akerite group. 



The contact form of this Gray Butte syenite is fine-grained 

 and resembles the dike rocks of the district. The actual con- 

 tact was not observed owing to the great amount of talus, but 

 the contact form and the flinty, hardened shales showed its 

 proximity. The rock from the contact is very fine-grained 

 and dense; it appears exactly like the ground-mass of the 

 main type and has the same pale grayish-green color. The 

 most noticeable feature is the almost entire lack of the large 

 feldspar phenocrysts which in the main type are so thickly 

 crowded together and which make so characteristic a feature 

 of the main rock. Occasional feldspar phenocrysts occur, but 

 they are rare, mostly quite small in size, and do not present 

 very sharp boundaries. The little black prisms of augite are 

 the same in size as before, but somewhat less in quantity. 



Under the microscope the contact form is precisely like the 

 main type previously described ; it consists of the same miner- 

 als — segirine-augite, alkali feldspars, and quartz. The struc- 

 ture is the same only the grain is somewhat finer. The only 



