102 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



is commonly seen, under the microscope, to be replaced by qnartz in fine mosaic. 

 A less common result of metamorphism is the generation of small but beauti- 

 fully crystallized, idiomorphic plagioclase feldspars. Usually not more than 

 one of these new crystals is developed in one of the ' eggs.' Optical tests seem 

 to show that the feldspar is not the expected pure lime feldspar but the sodifer- 

 ous labradorite, with possibly the more basic bytownite sometimes developed. 



About 100 feet above the oolitic bed a similarly persistent zone of red 

 shale, about 60 feet thick was observed on the ridge of Mt. Hefty and at the 

 Boundary line on the high ridge immediately east of Wigwam river. 



Much the greater volume of the formation, practically all of it except 

 the relatively insignificant intercalations just noted 1 , is made up of the silicious 

 metargillite, which merits a few words of detailed description. 



The microscope shows that the rock was originally a typical argillaceous 

 sediment. More than half of it was clayey matter, in which small, angular 

 grains of clastic quartz, microcline, microperthite, and plagioclase were 

 embedded. In its present condition the rock carries a highly variable amount 

 of sericite, chlorite, and cryptocrystalline silica. There are, thus, all transi- 

 tions from partially recrystallized argillite to true metargillite. The specific 

 gravity correspondingly varies, from values as low as 2-6'87 to those as high as 

 2-754, the latter representing practically complete crystallinity with abundant 

 sericite developed. The average specific gravity of seven selected specimens 

 is 2-722. 



In several of the thin sections grains of carbonate, causing liberal efferves- 

 cence with cold dilute acid, are to be seen distributed through the silicious 

 matrix. From the fact that a half dozen or more of these grains (calcite, 

 siderite, or ankerite?) extinguish together on rotation between crossed nicols, 

 it may be concluded that they are of secondary origin, and, like the sericite, 

 crystallized after burial of the sediment. At the same time there is no reason 

 to doubt that the material of the carbonate was a component of the original 

 mud. 



Professor Dittrich has chemically analyzed one of the least recrystallized 

 phases. The specimen (No. 1250) was collected at the top of the 6,700-foot 

 summit about 2,300 yards southwest of the Boundary monument at Wigwam 

 river. The analysis yielded the following proportions : — 



Analysis of type specimen, MacDonald formation. 



SiOo 68-37 



ALA 7-02 



Fe„0 3 4-41 



Feb 3-99 



MgO 4-41 



CaO 3-89 



NjuO -87 



K„0 1-34 



H" 2 0' at 110°C -25 



H,0 above 110°C 3-60 



C0 2 1-91 



100-06 

 Sp.gr 2-687 



