314 KEMP AXD KNIGHT — LEUCITE HILLS OF W^O^riXG 



250 to 300 feet in a distance apjiroximately 2 miles. Again southwest of ■ 

 number 5 the sheets rise to the height of number 5 within three-fourths 

 of a mile. Number 5 is 225 feet by aneroid above the mesa at its base. 

 The rim at this point is not so high, however, as cone number 4. 



COXES 



There are five cones arising from the surface of the mesa. We have 

 numbered these, beginning with the eastern one, numbers 1 to 5. The 

 statement in volume 8. page 175, of the Bulletin that there are six is due 

 to the supposition that Zirkel and Emmons mesas were connected. 



Cone number 1 is not a complete cone, but a half circle opening to the 

 west. It is 210 feet high and consists of fragments of pumice. An illus- 

 tration of it viewed from the east from a distance of a mile or more ap- 

 pears in figure 2, plate 14, of volume 8 of the Bulletin. Soon after the 

 elevation was determined the barometer gave the reading 7,250 feet at 

 Fifteen-mile spring. At the base of the flow it read 7,525 feet, at the 

 base of number 1 cone 7,640 feet, and the crest of the cone 7,850 feet. 



Cone number 2 is more conical and less evidently a remnant of a ring. 

 It is chiefly, if not entirely, made up of fragments of pumice. It rises 

 about 250 feet above the mesa. The slope of its surface is 17 to 18 

 degrees. 



Numbers 3 and 4 lie southwest of number 2 and a mile or more dis- 

 tant. Number 5 is a mass of solid lava. 



We take pleasure in naming this mesa after Professor Ferdinand 

 Zirkel, of the University of Leipzig, who was the first to recognize the 

 leucite in these rocks and to whom American petrographers in particular 

 owe so great a debt for his petrographic report on the rocks collected b}^ 

 the geological survey along the fortieth parallel. 



PETROGRAPHY 



Twelve thin-sections have been prepared of the rock of Zirkel mesa, 

 distributed so generally over its area that there is little chance of new 

 varieties appearing. Eleven of the twelve proved to be orendite, the 

 remaining one being w3^omingite. Our wA^omingite w^as obtained at the 

 extreme western end, and is the rock described as the rich leucite rock 

 b}^ one of us and figured in the earlier paper.* A photomicrograph is 

 reproduced as figure 1 of plate 38. This rock also contains the yellow- 

 ish hornblende described b}'' Doctor Cross, although in his original 

 wyomingite this particular constituent did not happen to occur. It was 

 overlooked by one of us (J. F. K.) in preparing the previous paper, its 



* Bull. Geol. Soc. Am., vol. S, p. 177. fig. 2. 



