Calkins. I Petrography of the John Bay Basin. 



157 



The contraction parting of this rock, as observed by the 

 writer in the field, is rather curious. It may be characterized as 

 a sort of perlitic structure on a large scale. Blocks of all sizes, 

 from that of a sugar barrel down, bounded by intersecting, 

 curved surfaces, are strewn on the slope beneath the crumbling 

 edge of the flow. Ditch block is subdivided and re-subdivided 

 by smaller and smaller systems of perlitic cracks, whose distinct- 

 ness generally decreases with their size. Under the blows of the 

 hammer, the rock breaks down so readily that it is impossible to 

 obtain trimmed hand-specimens of the usual form. The broader 

 surfaces of contraction parting are often coated with a thin film 

 of milky opal, decorated with delicate moss-like figures in black, 

 which are probably formed of oxide of manganese. 



The rock, macroscopically examined, appears of a rather dark 

 pearl-gray color. It is essentially composed of glass. A smooth 

 surface examined with the aid of a lens, exhibits a fine eutaxitic 

 structure, the wavy streaks being alternately gray and opaque 

 and dark and transparent. 



Under the microscope accessory magnetite, apatite and zircon 

 were also noted, and a very few small phenocrysts of ortho- 

 rhombic pyroxene of rather deep green color. 



The feldspar phenocrysts, as in the lithoidal rhyolite, are 

 mainly anorthoclase, recognized by the characteristic shadowy 

 striation and the small angle of the optic axes. In addition there 

 may be orthoclase, — though none was proved, — and one crystal 

 with distinct and continuous twin lamellae was seen, which is 

 probably an acid plagioclase. 



The flow- structure of the grouudmass is beautifully shown in 

 thin section, the alternating wavy bands appearing by ordinary 

 transmitted light to be cloudy gray and pale transparent brown. 



The lines of flow curve around the phenocrysts and a few 

 augular fragments, evidently derived by the breaking up of a thin 

 temporary crust formed on the fluid lava. 



The transparent laminae exemplify typically a phenomenon 

 briefly noted in two of the other John Day lavas. This supposed 

 glass, carefully examined between crossed nicols, is found to be 

 not entirely isotropic. Radiating from numerous points on the 

 edges of the laminae into the clear substance are obscure dark 



