PROCEEDINGS OF THE NATIONAL MUSEUM. 665 



beeu lionibleudes. Subjected to chemical tests the rock yields 2.5 per 

 cent, of potash (K2O). 



Lamprophi/res. — Between South Boulder and Auteloi)e creeks. The 

 intrusives here are a gray to j)inkish micaceous syenite and a dark gray,, 

 basic porphyritic rock immediately overlying it. Both are intrusive in 

 Cretaceous sandstones. The basic rock (No. 02409, U.S.N.M.) occurs in a 

 sheet apparently fifteen to twenty feet in thickness, though this could not 

 be determined for a certainty owing to lack of exposures. This rock in 

 the hand specimen shows a gray and apparently crystalline grouudmass 

 thickly studded with deep greenish black very perfectly formed augites 

 and olivines and very numerous minute ilecks of brown mica. The 

 augites in extreme cases are ten mm. in length and half as broad; forms 

 five mm. in length are common. 



As seen under the microsco])e and with a ))ower of eighty diameters 

 the rock presents a colorless feldspathic, holocrystalline (?) ground- 

 mass, carrying scattering granules of iron ore, numerous greatly elon- 

 gated dusky apatites, a few small augites, abundant elongated and very 

 irregularly outlined shreds of brown, strongly dichroic mica and the 

 porphyritic augites and olivines above noted. The rock is beautifully 

 fresh and unaltered. 



The pori)hyritic augites show very perfect crystal outlines of the 

 ordinary typej twin forms are rare. Inclosures are minute and limited 

 to what is apparently portions of the grouudmass, iron oxides, and 

 mica scales; a faint zonal structure is sometimes ajtparent. The 

 olivines are also at times in very perfect crystal forms, though more 

 frequently rounded witli extremely irregularly toothed or etched 

 outlines closely bounded by small shreds of the brownish mica. This 

 feature is likewise occasionally sliown by the augites. The mica itself 

 never shows hexagonal outlines, but is always in very irregular and 

 greatly elongated folia. 



The grouudmass. — Revolved between crossed nicols no portion of the 

 field remains entirely dark, but breaks up into irregularly bounded 

 areas showing at times an almost granular structure, but nioie com- 

 monly one imi^erfectly columnar-radiating, the dark wave merging from 

 one ])ortion to another, and in few cases showing crystal outlines- 

 sufficiently well defined for determination. Occasional elongated forms 

 show a maximum extinction paraded and at right angles with the axis, 

 of elongation. In rare instances still others occur showing twin striae 

 characteristic of i)lagioclase feldspars, but beyond this the microscope 

 fails to give satisfactory results. No interference figures are obtaina- 

 ble nor are cleavage lines apparent. An attempt was made at sepa- 

 rating the minerals of the grouudmass by means of specific gravities,, 

 but results were not particularly satisfactory owing to inclosures of 

 mica and iron ores. After repeated attempts a small amount coming 

 down at 2.0 and showing under the microscope no admixture of augites,. 

 olivine, or iron ores was obtained, which yielded me on analysis as below. 



