134 REPORT rxiTi'.n STATES geological .survey. 



It is about 250 feet high, has a very narrow base, and forms a prominent 

 feature in the landscape. Southeast of the mountain we observe a num- 

 ber of similar buttes, which approximately follow a course of east 45° 

 south. Most of them are table-topped hills, from 600 to 900 feet in height, 

 showing volcanic caps of varying thicknesses. They may be estimated 

 at 200 to 700 feet. 



More interesting than the table-topped hills was one occurrence at 

 Basalt Springs. There a ridge, trending south of east and composed of 

 Wasatch beds, was covered with an extensive tlow of basaltoid rock. 

 On the summit of the ridge, three cone-shaped hills were noticed 

 standing in its strike. One of these presented the aspect of a crater, 

 while the other two resembled the highest point of. Mount Essex. Along 

 the northern slope of the ridge the volcanics were broken off abruptly, 

 and tumbled down the side in most chaotic masses. Huge bowlders, 

 weighing hundreds of tons, were piled on top of each other, and coin- 

 posed the slope directly below the vertical face of the rock in situ. It 

 was found that a process of undermining has caused a removal of 

 Wasatch beds, and that the overlying lava had fallen down into the exca- 

 vation thus produced. Near the unbroken northern edge of the ridge 

 the last fissure formed by this separation was observed. It extended in 

 a nearly straight line for several hundred yards, was over 40 yards deep, 

 and not more than 20 feet wide at the top, practically closed below. The 

 movement of this huge mass was evidently arrested by the accumulated 

 bowlders on the slope a little lower down. 



These buttes and hills furnish an indication of the extent of erosion 

 which took place in this region. , Through fissures having a strike paral- 

 lel among themselves, the lava was ejected and poured over the level of 

 the Wasatch, and, in part, Laramie strata. Probably the several areas 

 of eruption were disconnected from the beginning. Fluviatile erosion, 

 shaping its course in accordance with the horizontal distribution of this 

 hard rock, cut deep valleys along such places where it could successfully 

 attack the beds. By undermining the volcanics a diminution of their 

 area was achieved and they were forced to assume the peculiar forms 

 that at present characterize them. Tendency to columnar structure, or 

 the pronounced development thereof, greatly aided the formation of ver- 

 tical faces in the lavas. After the masses had fallen, their removal and 

 transportation were effected by the same agents that excavated the val- 

 leys. In the case of the Basalt Springs ridge they were individually 

 too huge to be attacked, therefore retain their original position. 



Upon critical examination we find that the volcanics of these hills 

 show highly interesting features. Some of them are gray and grayish- 

 brown, showing a microcrystallinegroundniass, with no visible segregated 

 mineral but biotite ; others are decidedly basaltic and contain olivine. 

 During the progress of the Fortieth Parallel Survey this region was 

 visited and the rocks subjected to microscopical examination. The first- 

 mentioned variety proves to be a leucitophyrite. * It appears that the 

 groundmass is composed entirely of minute crystals of leucite. This 

 occurrence is one of great interest, inasmuch as it forms the first one 

 of this character on the North American continent. In Europe, the 

 leucite rocks have attained great celebrity on account of their unique 

 character as well as on account of the beautiful crystals they contain. 



By the discoverers of the peculiar mineralogical constitution of these 

 eruptives, the entire series of hills has been named "Leucite Hills." At 

 Mount Essex I observed the most typical leucitophyrite. It seems to be 



* U. S. Geol. Surv. Fortieth Par., vol. vi, p. 260. 



