68 DEPARTMENT OF THE INTERIOR 



2 GEORGE V„ A. 1912 



orientated crystals in the exceedingly fine-grained rock. Many fine shreds and 

 thin scale? of similar material with needle-like cross-sections, have all the 

 optical characteristics of sericite, and it seems highly probable that it is this 

 mineral which forms the base of the dominant rock. 



A whole thin section may, then, be made up of a homogeneous, intimate 

 mixture of quartz and sericite with accessory grains of iron oxide; or, as is 

 more commonly the case, the slide shows a well-defined banding representing 

 original bedding. In the latter type of section the bedding is marked by alter- 

 nation of more quartzose and more sericitic material or, yet more clearly, by 

 long lines of limonitic and carbonaceous particles. A few grains of ilmenite 

 or magnetite seem never to fail, but no other accessories, such as feldspars, have 

 been observed. The specific gravity of four specimens, taken to represent the 

 average types of these quartz-sericite rocks and thus the greater volume of the 

 whole formation, varies from 2-708 to 2-760, with a mean value of 2-740. 

 This comparatively high density shows that the sericite is fairly abundant. 



The almost complete recrystallization of the original rock is evidenced in 

 the intimate interlocking of the quartz and micaceous mineral and in the 

 entire absence of amorphous argillaceous matter. The sericite is slightly move 

 developed in the bedding plane than elsewhere, thus somewhat aiding the fis- 

 sility of the rock in those planes. On the other hand, many scales of this 

 mineral have their longer diameters developed at high angles to the bedding 

 planes. Rarely is there a marked sheen on any surface of a hand specimen, 

 nor has true schistosity been developed except in a few very local areas. The 

 recrystallization of the typical rock is clearly the result of slow molecular 

 rearrangement incident to age-long deep burial without true dynamic rneta- 

 morphism. 



Although evident only after microscopic study, this change is so pronounced 

 that it is scarcely correct to speak of the normal phase of the Appekunny 

 as an argillite at all. It is as much a crystalline rock as is a granitoid gneiss. 

 We shall see that the same difficulty of nomenclature adheres to the description 

 of many thousands of feet of beds, in each of the Lewis, Galton, and Summit 

 series. It is convenient to have a term to represent these argillites, recrystal- 

 lized, yet neither hornfelses (due to contact metamorphism), nor true mica 

 slates or schists (due to dynamic metamorphism with development of notable 

 cleavage or schistosity, generally cutting across bedding planes). For such once- 

 argillaceous rocks, recrystallized merely by deep burial the name ' metargillite ' 

 will be used in the present report. 



The meaning of the term may be made clear through a comparison with 

 the names now in general use for pelitic rocks. An unconsolidated pelite is 

 a clay or mud. If consolidated but not extensively recrystallized, it is an 

 argillite. A thin-bedded, unaltered argillite which readily splits along the bed- 

 ding planes because those are planes of original weakness, is a shale. If recrys- 

 tallized during dynamic metamorphism only to the extent that easy cleavage 

 following the planes of similarly orientated microscopic mica plates is developed, 

 the rock is a slate. If an argillite becomes phanerocrystalline and foliated 



