1912] 



Baker: Western El Paso Range 



125 



with the upper surface of the basalt. Gilbert's original inter- 

 pretation of two interbedded basalt flows, as shown in his cross- 

 section, 6 is the correct one. 



These basalts are quite vesicular, the vesicles being partially 

 or entirely filled with quartz, chalcedony, or natrolite. Three or 

 four miles to the east of Red Rock Canon the basalt flows, which 

 have gradually become thinner, entirely disappear. 



Above the upper basalt flow in the Red Rock Canon locality 

 are found in upward succession: (1) a light bluish gray arkose 

 of coarse granite and lava breccia, interbedded with fine, green, 

 velvety tuff; (2) about 40 feet of rather fine, well-cemented 

 breccia; (3) 50 to 75 feet of ashy tuffaceous beds, quite coarse 

 and poorly assorted; (4) light brownish tuffs capped by two 

 layers of darker, reddish-brown, more resistant, fine breccia, com- 

 posed of the usual angular arkosic material and forming vertical 

 cliffs some of which are 50 feet in height. 



In Last Chance Gulch the basal beds are succeeded at 150 

 to 250 feet above their base by a red breccia containing dark 

 brick-red lava fragments in a matrix of dark pink tuff. The lava 

 fragments are mostly rather small, varying up to two inches or 

 more in diameter. They contain moderately abundant small 

 phenocrysts of fairly fresh oligoclase feldspar and a few pheno- 

 crysts of what was probably hornblende now altered to an aggre- 

 gate of iron oxides and chlorite. Some of the feldspar is altered 

 to kaolin. Magnetite is found in small grains. The feldspar of 

 the matrix is also oligoclase. The fragments have the same com- 

 position as their matrix and the rock is an andesitic tuff-breccia. 



The andesite tuff-breccia is overlain by 100 feet of fine, whitish 

 tuff-breccia containing small angular fragments of plutouic and 

 volcanic rocks and pieces of white fibrous pumice. The pumice 

 is quite abundant at this locality. Above come bluish-gray tuffs 

 similar to those already described in the Red Rock Canon section. 



The highest beds of the Rosamond which were examined are 

 exposed about four miles north of Ricardo post-office. These 

 higher beds are in general finer and not so well indurated as 

 those lower down in the series. In contrast with the badland 



e Geog. and Geol. Expl. and Surv. west of the Hundredth Meridian, vol. 

 3, p. 142, 1875. 



