316 KEMP AND KNIGHT — LEUCITE HILLS OF WYOMING 



III color the rocks vary all the wa}^ from a rich cream color, through 

 light to dark reddish brown, to dark gray. In texture they range from 

 the light porous pumice of the cones, through very vesicular amygda- 

 loids, to dense platy varieties resembling typical phonolites. On frac- 

 tured surfaces the amygdaloids have a waxy appearance, very like some 

 phonolites. Nearl}'- all of our hand si:)ecimens contain inclusions of the 

 rock, through which the lava has reached the surface. 



Emmons Mesa 



This was probably once the western end of Zirkel mesa. It is a rough 

 parallelogram in sha])e with a red cone at its northwestern corner (see 

 plate 39, figure 1). The cone is a prominent landmark and from its 

 peculiar color ma}" be easily distinguished from all points of view. In 

 other respects Emmons mesa appears to be much the same as the neigh- 

 boring Zirkel mesa, from which it has been separated by the erosion of 

 the intervening gulch. There seems to be entire correspondence along 

 the sundered escarpments, and it is probable that some subterranean 

 watercourse led to the collapse and removal of the lava which has dis- 

 appeared. 



We take pleasure in naming this mesa after Mr Samuel Franklin 

 Emmons, of the U. S. Geological Survey, who first collected the rock of 

 the Leucite hills. 



One specimen has been gathered by us from Emmons mesa, which 

 proves under the microscope to be a typical wyomingite, but we can not 

 say that the entire mesa is of this character. 



OsBORx Mesa 



This is aloft}^ and commanding mesa not of great superficial area. 

 The mesa is shaped somewhat like a short-footed stocking or sock, with 

 its long dimension running north 40 degrees west, magnetic. At the base 

 on the north side the aneroid recorded 7,760 feet and the angle of slope was 

 15 degrees up to the escarpment, which then arose nearly vertically. The 

 top was estimated at 400 feet above the gulch. Our specimens were 

 gathered from the talus, on the northwest side. 



Under the microscope the rock from Osborn mesa proves to be a typical 

 orendite. Leucite is present in moderate amount, sanidine is abundant, 

 while diopside, a little hornblende, and phlogopite make up the other 

 principal constituents. Deep, mahogany-colored rutile is conspicuous 

 in our slide. 



We take pleasure in naming this mesa after Professor Henry Fairfield 



