W. Cross — Igneous Books in Wyoming. 117 



This mesa, caused by the leucite-bearing lava, has a gently 

 undulating surface of bare rock in many places, with scanty 

 vegetation here and there, the sage bush being most common. 

 Several small cones rise above the mean level. There are six 

 of these cones, according to Kemp, who gives illustrations of 

 some of them in his paper. On the eastern side of the mesa, 

 in one of the indentations, is a spring, known to my teamster 

 and to the stockman who lived there as the " 15-mile spring." 

 Kemp refers to it as " the spring 10 miles from the railroad at 

 Airnond Station." [n the vicinity of this spring, which was 

 my headquarters and where the specimens for the Educational 

 Series were collected, the lava of the scarp is somewhat varia- 

 ble in texture, the greater part being vesicular in some degree, 

 with massive rock occurring here and there. It is in regard to 

 the relation between this massive variety, which corresponds 

 most closely to the type described by Zirkel, and the porous 

 form, that my field observations are unfortunately so imper- 

 fect. But little of the massive rock was seen, and then noth- 

 ing was observed to indicate that the two types belonged to 

 different flows. On this account, and from the chemical iden- 

 tity of the two rocks, I am at present inclined to regard the 

 leucitite of Zirkel's report as a part of the same flow that is 

 predominantly a more or less vesicular sanidine-leucite rock, 

 described in succeeding pages as orendite. 



I did not explore the mesa except near the eastern end, 

 where the rock of the surface was markedly porous and in 

 places nearly a pumice. Two of the cones were visited, and 

 my notes record that these consisted mainly of pumice. This 

 statement is at variance with that of Kemp that all these cones 

 are of solid rock and due, in his opinion, to the welling-up of 

 lava above volcanic conduits. It is of course possible that the 

 cones are not all of the same character. 



Oibtlying duties. — Nor th-of -east and about two miles from 

 the mesa is an outlier, known as Spring Butte to my local 

 authorities, but described by Kemp as Orenda Butte, a name 

 also found on the Land Office map. The inclusions noticed in 

 the lava at this point will be specially described in a later 

 section. 



A few miles north of the principal mesa is a remnant of the 

 former sheet in what is known as North Table Butte. We 

 visited this point, reaching the summit through a cleft in the 

 scarp on the northern side. The butte rises about 750 feet 

 above the valley at its northern base, but only some 50 feet of 

 lava is present. The rocks of the scarp vary as in the mesa to 

 the south, but are lighter colored. Pumiceous material was 

 found on the top of the butte. Its summit is considerably 

 above the level of the mesa to the south, but whether this is 



Am. Jour. Sci.— Fourth Series, Yol. IY, No. 20. — August, 1897. 

 9 



