156 C. L. WHITTLE METAMORPHIC CONGLOMERATE. 



and feldspar grains and by the development of serecite, muscovite, biotite 

 and magnetite. These evidences of alteration are only incipient, so that 

 the genuine clastic character of its principal constituents is undoubted. 

 Quartz pebbles are most numerous and some attain dimensions suffi- 

 ciently large to be dignified by the term bowlders, being 18 inches in 

 maximum length, and make up nearly 90 per cent of the detrital material. 

 Gneiss and feldspar pebbles are also common, pebbles of the latter in 

 rare instances having dimensions of two by three inches. The ground- 

 mass is fine granular quartz, magnetite, plates of muscovite, and prisms 

 of sericite. Under the microscope the evidence of the clastic nature of 

 the rock is emphasized by the water-worn outlines of its pebbles and their 

 parallel arrangement due to gravity. 



Secondary Enlargement of clastic Tourmalines. — Thin sections of the con- 

 glomerate (numbers 699 and 700) were studied in detail and their de- 

 scriptions are given later. The tourmalines occur in the rock as minute, 

 stout crystals, bounded by crystalline faces developed independently of 

 any apparent nucleus, and as imperfectly bounded areas of a secondary 

 origin deposited upon clastic nuclei of the same mineral. Figure 1, 

 plate 2, which is an instance of secondary enlargement of an original 

 clastic grain, is given to show development of prismatic planes, and also 

 two terminal faces of a rhombohedron imperfectly developed. The 

 boundary of this nucleus is not perceptibly water-worn, but in other ex- 

 amples of this phenomenon the nuclei are distinctly pebbles of attrition, 

 oriented with the other detrital constituents. The clastic tourmaline is 

 colorless in transmitted light when the plane of the polarizer coincides 

 with the extraordinary ray, and orange-yellow when the plane of the 

 polarizer coincides with the ordinary ray. It gives in converging light 

 one arm of a cross. The surrounding mineral is separated by a distinct 

 line from the core and is much darker-colored, as might be expected 

 from the isomorphous nature of the mineraL The pleochroism of the 

 rim is from brown to nearly black, and there are corresponding differ- 

 ences in the intensity of the interference colors ; the center polarizes in 

 brilliant greens and reds, while the interference color of the rim is masked 

 by the bod}'' color. The secondary tourmaline is perfectly oriented with 

 the core, and parallels the secondary enlargements of quartz grains de- 

 scribed by Irving and Van Hise* in this respect. There are abundant 

 inclusions' of iron oxide (possibby ilmenite) and some quartz in the sec- 

 ondary part, but the core is free from them. They are particularly 

 numerous about the sides of the angular core and along the basal por- 

 tion of the enlargement. Two corners of the latter have inclusions of 

 pale-green sericite prisms projecting into the groundmass, showing the 



♦Bulletin No. 8, U. S. Geol. Survey, issi. 



