calkins.] Petrography of the John Bay Basin. 



153 



minerals recognizable by the unaided eye. With a lens, however, 

 it may be seen that the rock usually contains numerous lenticular 

 opening's lined with quartz. Larger cavities lined with milky 

 opal are occasionally present. Certain phases are character- 

 istically marked by wavy and more or less parallel lines of 

 brownish red color. These are the traces on the broken surface 

 of contorted laminae impregnated with ferric oxide. 



In the thin section on the stage of the microscope, we find 

 magnetite in a few large grains and zircon and apatite as stout 

 prisms in the groundmass or small crystals included iu feldspar, 

 all being in rather small amount. Original quartz is also 

 found, but it is confined to the groundmass, and is usually 

 inconspicuous. 



The feldspar phenocrysts are usually sharply idiomorphic. 

 The average relative development in the direction of the crystal- 

 lographies axes may be roughly indicated by the proportion 

 a:b:C : : 4:2:3. The feldspar is always perfectly fresh, and 

 carries but few inclusions of zircon, apatite and glass. Between 

 crossed nicols they are seen to be without zonal structure. Both 

 albite and pericline striations seem to be present in a majority of 

 the crystals, while Carlsbad twins occur occasionally in a small 

 proportion of the crystals. By optical methods of investigation, 

 the feldspars were found to be sanidine and anorthoclase. The 

 latter is decidedly predominant. As sanidine only a single 

 crystal was certainly proved; this crystal, which showed no 

 twinning, was cut nearly normal to the bisectrix a, and its optic- 

 axial angle was almost zero. There are, however, a sufficient 

 number of untwinned sections to warrant a presumption that 

 more orthoclase occurs. In the triclinic feldspar the striae are 

 usually narrow and always very indistinctly defined. In sections 

 cut nearly perpendicular to a, that axis is found to bisect a very 

 acute optic axial angle, so that the black hyperbolae of the 

 interference figure, when that is well centered, never pass out of 

 the field during a complete revolution of the stage. The extinc- 

 tion angle in these sections could not be sharply determined, and 

 the extinction angle in the section perpendicular to C could not 

 be determined accurately. A crystal giving a slightly eccentric 

 obtuse bisectrix-figure extinguished 8° from the basal cleavage, 



