Calkins.] 



Petrography of the 'John Day Basin. 



141 



fuses. From the hydrofluosilicate solution, were obtained good 

 sodium crystals and some amorphous matter. 



Iu the decomposed matrix of the large spherulites, opal 

 occurs iu abundance. This material is ofteuest milky, aud of a 

 yellowish or bluish white, but many specimens more transparent 

 exhibit considerable play of color. A probably similar occurrence 

 in another portion of the State has been exploited for precious 

 opal, but was found unprofitable and abandoned. 



Bhyolites of Clarno's Ferry. — The specimens of rhyolitic lava 

 and tuff brought from this locality seem to represent two or three 

 chemical types. 



One of the massive specimens appears to be a typical liparite, 

 with few characters of especial interest. The phenocrysts are 

 quartz, sanidine, and oligoclase, and lie in a reddish purple 

 groundmass, devitrifled and showing in part a spherulitic structure. 

 There are a few inclusions of decomposed hypersthene andesite, 

 which give evidence as to the succession at this locality. 



The other lava is of more unusual character. Macroscopic-ally, 

 it shows abundant phenocrysts of fresh glassy feldspar and quartz, 

 in a cream-white, lustreless groundmass. This groundmass has 

 been decomposed locally in a peculiar manner, for the hand- 

 specimen contains several small masses of a soft kaolin-like 

 substance, with imbedded crystals of quartz and glassy feldspar, 

 which can be accounted for only by such a decomposition. 



Under the microscope, accessory apatite, magnetite and zircon 

 are noted. The groundmass is partially de vitrified; it is seen in 

 polarized light to be specked with minute, doubly refracting 

 grains, and a feebly developed mierospherulitic structure may be 

 observed locally on careful examination. The bipyramids of 

 quartz have suffered the inevitable magmatic corrosion. A dis- 

 tinct rhombohedral cleavage was observed in one crystal. The 

 feldspars, which occur in well-formed crystals, comprise both 

 orthoclase and plagioclase, but the latter is decidedly predom- 

 inant. They inclose apatite, magnetite, and zircon, contrasting 

 with the quartz, which is devoid of mineral inclusions. No 

 ferromaguesian silicates are present in the rock. 



The measurement of extinction angles for the determination of 

 the feldspars, which was carried out with some care, led to rather 



