NO. n73. EOCKS FROM UNITED STA TES-MEXTCO BOUNDARY— LORJ). 779 



oidal surface forms and a bluish gray, more coarsely crystalline rock 

 mass confined chiefly to the interior portion of the nodule, and consist- 

 ing of tine prismatic crystals of orthoclase, allotriomorphic quartz, and 

 minute particles of magnetite and biotite. These feldspar needles are 

 optically positive and elongated parallel c, with symmetrical position 

 ol the plane of opti(;al axes. They show weak double refraction and 

 extinguish at an angle of not more than 5°, and have either a divergent 

 tibrous structure, with at times a slight tendency to fluidal arrange- 

 ment, or are intergrown poikilitically with quartz, as was the case in 

 the coarser grained areas of the rhyolite from the San Barnardino 

 River (No. 343) (p. 778). 



The denser spherulitic aggregates forming the surface of the nodules 

 consist principally of optically positive feldspar libers with a few tilms 

 of biotite; (juartz, if present, is too finely distributed to be recognized 

 by optical tests. 



Owing to unequal development in different directions these denser 

 portions of the nodules assume all manner of irregular discoidal and 

 plume like forms, frequently overlapping and interpenetrating in such 

 a manner as to render it difficult to determine where one ceases and 

 the one adjoining begins. 



The brown color of the nodules is due to the oxidation of trichites 

 and minute lerritic particles that intersect them in parallel lines regard- 

 less of their complex inner structure. 



The chemical composition of these peculiar types of compound sphe- 

 rulites is given under Analysis No. I, and differs not essentially from 

 that of the dark blue spherulites from Obsidian Clili" as given by 

 Iddings ' (No. II). They contain somewhat less ISiO^ and more Fe203. 



Analysis of spherulites. 



Constituents. 



SiOj 



AI2O3 



Fe-iOa 



CaO 



Na,0 



K2O 



Ign 



Total 



No. I. No. II. 



74.75 

 12.01 

 .3.72 

 .54 

 4.02 

 4.73 

 .52 



100. 29 



76.70 



12.30 



1.43 



.39 



3.89 



4.73 



.66 



100. 10 



/. Hornblende-micaandesite. — Fifty-eight miles west of the Rio Grande 

 River, Mon. No. 19 (No. 53288). Corner Mou. Apache Mountains, Mon. 

 No. 40 (No. 53289). Dog Spring, Dog Mountains, Mon. No. 55 (No. 

 53290). Tule Mountains, Mon. No. 180 (No. 53291). San Isidro Ranch, 

 north of Signal Mountain, Mon. No. 224 (No. 53292). Coast Range, 

 Mon. No. 224 (No. 53293). 



These are fine-grained porphyritic rock types, varying in color from 

 steel gray to reddish brown according to the state of preservation. 



'J. P. Iddings, Obsidian Cliff, Yellowstone National Park, Seventh Ann. Rept. 

 U. S. Geol. Surv., Ill, 1885-8G, p. 282. 



