1894, 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



649 



During the season of 18SC Dr. Peale brought in from the northwest 

 side of the lower valley of Cottonwood Creek, and labelled as from the 

 " Upper dike," a small specimen of badly weathered, fine grained, light 

 gray rock, thickly studded with small folia of black mica and minute 

 augites. Under a low power the rock appears almost holocrystalline 

 and composed of lath-shaped, short, stout Interlocking feldspars, light 

 greenish augites, and scales of brown mica. The feldspars are all 

 muddied through decomposition and optical determinations are very 

 unsatisfactory. A ijortion of them show twin striie; others show none 

 and are presumably in part sanidiu. 



The augites are all small (one-third mm. in greatest diameter), and as 

 a rule in imperfect and fractured forms. Cross sections, however, fre- 

 quently show quite perfect outlines. They are very light greenish in 

 color in the section. The mica is reddish-brown, strongly dichroic, 

 and occurs in irregular shreds, in very perfect hexagonal tablets, as a 

 narrow border about the iron ores, and in a few instances was observed 

 a like border about elongated augites, 



A high power shows the interstices of the feldspars occupied by a 

 colorless isotropic substance or a very light green chloritic material 

 evidently derived therefrom. When an uncovered slide is treated with 

 hydrochloric acid there are shortly produced abundant cubes of sodium 

 chloride. So abundant were these cubes that careful search was made 

 for nepheline or sodalite, but with unsatisfactory results. The cavities 

 left in the slide after treatment with hydrochloric acid presented in no 

 cases the outlines of any crystallized mineral, but are in all cases 

 irregular area^^ scattered promiscuously throughout the mass of feld- 

 si)ars. For the time being the true nature of the isotropic mineral 

 which gave rise to these was a mystery, but in the light of subsequent 

 observations there seems little doubt but that they are of sodalite and 

 the rock a phase of the mica and augite bearing syenitic lamprophyres, 

 described later (p. 671). A partial analysis of the rock yielded Mr. 

 Eakins results as follows: 



SiO.2 . 

 Al,03 

 Fe^Oa 

 CaO . 

 MgO, 

 K,() . 

 Ka,0 



Per cent. 



54.29 

 18.47 

 5.67 

 3.69 

 3.98 

 5.92 

 4.13 



90. 15 



Augite porpliyrite. — Intrusive sheet some sixty feet in thickness just 

 above Horse Shoe Bend of the Missouri River. In strike and dip it 

 follows the Cretaceous sandstone in which it lies, cutting across the 

 beds only very slightly, if at all. It is well exposed in the bluffs on 

 the west side of and facing the river. Both upper and lower contact 

 are here readily found. 



