318 KEMP AND KNIGPIT LEUCITE HILLS OF WYOMING 



studies we so largely owe our knowledge of the detailed petrography of 

 these lavas. 



Hatcher Mesa 



This is located about a mile north of the Red cone on Emmons mesa 

 and is a conspicuous mesa with a high vertical scarp and a nearl}^ flat 

 top, which is about a half mile in diameter. It is illustrated by figure 2 of 

 plate 39. Its elevation above the sea was not taken, but it, will not exceed 

 7,600 feet. In this instance the talus from the capping lava has reached 

 the valley which passes the southern base, having traveled a vertical dis- 

 tance of about 500 feet. While the rock looks very much like that from 

 the other mesas, it is, as a rule, more vesicular and also contains many 

 more inclusions and in greater variety than any other noted. Chief 

 among these inclusions were rounded fragments of granite or of some 

 ancient crystalline rock. 



This mesa has been named after Mr J. B. Hatcher, of the Carnegie 

 Museum, Pittsburg, in recognition of his valuable services in connection 

 with Wyoming paleontology. 



Under the microscope the rock of this mesa presents characters not 

 elsewhere seen. It is partly aegirite-wyomingite and partly aegirite- 

 orendite. The principal minerals of the former are leucite. diopside, 

 aegirite, hornblende, and phlogopite, while the latter has in addition 

 sanidine. The aegirite occurs in acicular crystals, of the usual green 

 color, of rather feeble pleochroism and nearly or quite parallel extinc- 

 tion. It also forms the ends of diopside crystals, as it is so prone to 

 do in other rocks. Its characters are the same in both the wyomingite 

 and the orendite. The latter exhibits beautiful sagenite nets in the 

 phlogopite. 



North Pilot Mesa 



This is the most conspicuous of all of the leucitic exposures, and has 

 been well named, for upon entering this region from any direction it is 

 truly a pilot, as it rears its truncated cap upwards of 1,000 feet above the 

 eastern base. From a distance the mesa appears to be nearly round, 

 but upon examination it proved to be elliptical, with the longer 

 axis extending northwest and southeast and being nearly a half mile in 

 length. The elevation, as determined with an aneroid barometer, is 

 7,750 feet. It is nearly surrounded with a vertical scarj), which varies 

 from 25 to 50 feet in height and is inaccessible, Avitli the exception of 

 the northeastern face, where the large talus blocks reach nearly to the 

 top of the mesa. From the surrounding country the surface appears to 

 be nearly level, but it is quite uneven, owing to the relative facility 



