PROCEEDINGS OF THE NATIONAL MUSEUM. 



651 



made out with any degree of certainty: c, very liglit greenisb; S, very 

 faintly reddish, and b, faint yellowish, scarcely at all reddish. The 

 mineral shows extinction in all cases parallel and at right angle with 

 the G axis; it is biaxial, negative, and sections cnt at right angles to 

 the a axis show the immergence of a bisextiix with the plane of the 

 optic axes in that of the a and c axes. Dispersion p'>v. These char- 

 acteristics alone are sufficient to demonstrate the true character of 

 the mineral. 



Mr. J. S. Diller, of the U. S. Geological Survey, has kindly loaned 

 me sections of the hyperstheiie basalt described by him from Mount 

 Thielsou, Oregon,* from an examination of which I am able to make 

 the following comparisons: the two rocks have essentially the same 

 structure, but differ in that the sample from lied Blufi' shows a rela- 

 tively smaller number of porphyritic plagioclases, a far larger pro- 

 portion of hyperstheiie, and also a larger proportion of plagioclases in 

 the groundmass, necessitating therefore a smaller proportional amount 

 of glassy base. The feldspars of the Oregon rock are much better 

 developed than in that of Eed Bluff and the "opacite" particles much 

 larger and more distinctly granular. Bulk analysis of the Eed Bluff 

 rock by Mr. Eakins yielded results as below. In column ii is given 

 that of the Mount Thielsou rock, permission for the use of which has 

 been kindly granted by Mr. Diller. 



From this it would appear that the rock is much more nearly related 

 to the andesites than the basalts, although on purely structural 

 grounds it seems more like the latter. 



Peridoiite, var. Wehrlite. — Hills three miles northwest of Eed Bluff. 

 This rock (Nos. 70675 and 73162, U.S.y.M.) occurs intrusive in the 

 gneiss and forms on the present surface only several small, nearly cir- 

 cular, inconspicuous outcrops, standing but a few feet above the sur- 

 rounding gneiss and broken into rough, angular blocks weathering 

 brownish. Two textural varieties are readily apparent. One, a some- 

 what coarse, distinctly crystalline rock, showing on fresh surfaces 



* Am. Jour. Sci., xxviii, 1884, p. 252. 



