820 KEMP AND KNIGHT LEUCITE HILLS OE WYOMING 



crystals or else surrounded by a rim of phlogopite, a not unnatural rela- 

 tion for two orthosilicates of the same normal, principal base, magnesia. 

 The phlogopite is often surrounded by crowns and borders of rutile, 

 which make a deep mahogany colored rim. Leucite is extremely rare 

 in some of these rocks, while sanidine is correspondingl}' abundant. Of 

 themselves the rocks make very good trachytes in their mineralogy, 

 being variations from the typical trachytes, chiefl}^ because of the ab- 

 normal abundance of diopside. 



North Table 



Northw^est of North Pilot and less than a mile away there is a mesa 

 with a surface as level as if it had been planed, but it is 235 feet below 

 the crest of North Pilot (see plate 41, figure 2). About it there is a quite 

 prominent scarp. There are also highly inclined talus slopes, such as 

 accompany all of the mesas. The top is between a quarter and a half 

 mile in diameter. The eruptive rock closely resembles that from other 

 localities, but the thickness of the sheet appears to be less than is usual. 

 We call this the " North Table " in distinction from "South Table," 

 whose description follows : 



Four slides have been prepared of the North Table, all from the north- 

 eastern portion. Three are orendite and one is wyomingite. The pecul- 

 iar yellow hornblende is ver}^ abundant and with its increase diopside 

 wanes. The wyomingite is a pinkish rock, while the orendites are gray- 

 ish brown. All are more or less vesicular. 



South Table 



This is the smallest mesa in the Leucite hills, being much less than a 

 quarter of a mile in diameter and 400 feet below North Pilot in elevation 

 (see plate 41, figure 2). There is a quite prominent scarp about it, and 

 the rocks, so far as examined, are largely schistose, the schistosity being 

 developed vertically. The rock is dense and platy, and appears to be 

 an intruded sheet, both in its structural relations and in its petrographical 

 characters. The vertical schistosity would suggest a vertical movement, 

 but this is hard to understand in a mesa. It is possible that the pressure 

 on the rock which was required to force it between strata caused the 

 phlogopites to arrange themselves across the line of thrust. 



Two specimens were gathered from the mesa, and both are typical 

 wyomingites. Leucites make up much the greater part of the rock, and 

 exhibit all the characteristic features of this mineral. They are of good 

 size and are excellent for study. Diopside and phlogopite are practically 



