Calkins.] Petrography of the John Day Basin 



1 1!) 



cleavage, at right angles to which is a rather indistinct parting. 

 In one specimen the lustrous cleavage lamellae show considerable 

 distortion, which is doubtless the result of mountain-making 

 stresses . 



Although maeroseopically the crystals appear to be homo- 

 geneous, they are at once seen, when examined in thin section, 

 to be lamellar intergrowths of orthorhombic and monoclinic 

 pyroxene. The former variety seems in the majority of grains 

 to predominate, but the proportions are in certain cases reversed. 

 A section parallel to the good cleavage of one crystal showed, on 

 examination in convergent polarized light , a single bar traversing 

 a system of colored rings. A second section, perpendicular to 

 the first and parallel to the imperfect parting, showed high 

 interference colors, and an extinction angle, measured from the 

 cleavage, of 39°. Parallel to these cleavage traces narrow stripes 

 were visible in polarized light, being distinguished by their low 

 interference colors and straight extinction. These were sections 

 of lamellae of a rhombic pyroxene, found by examination in 

 convergent light to be cut perpendicular to the bisectrix a. 

 More frequently, however, the sections cut parallel to the imper- 

 fect parting are found to consist mainly of rhombic pyroxene, 

 in which case the bisectrix 3 is normal to the section. Stripes 

 of the monoclinic mineral are always seen in greater or less 

 amount, and show an extinction angle near 40°. From the facts 

 just stated it is evident that the two minerals form regular inter- 

 growths after the usual plan, having the vertical axis in common, 

 and the orthopinacoidal faces of the monoclinic in composition 

 with the brachypinacoidal faces of the orthorhombic lamellae. 



The orthorhombic pyroxene has, in thin section, a very pale 

 greenish brown tint, and a feeble though evident pleoehroism. 

 It therefore is probably an enstatite or a bronzite poor in iron. 

 The monoclinic mineral, which appears almost colorless and 

 without pleoehroism, has the extinction angle of diallage, and 

 its characteristic lamellar parting; it is therefore referred to 

 that species. 



The inclusions, which are not especially abundant, are similar 

 for the two minerals. They consist mainly of opaque rods and 

 grains, arranged in lines parallel to the vertical axis of the host. 



