ZIRKEL MESA 313 



of Fifteen-mile s])ring. The Laramie beds dip about 5 degrees north- 

 west and strike northeast. The mesa has an abrupt escarpment which, 

 while more or less precipitous, is broken by small gulches which give 

 access to the top. In a few jjlaces it may be scaled by horses. The 

 escarpment is surrounded, as are indeed those of all tbe mesas, by a 

 heav}^ talus, which masks the lower contact. The edge at the northeast 

 corner is illustrated by figure 3 of plate 14, volume 8, page 174, of the 

 Bulletin. 



The mesa's long axis runs east and west a distance of about 5 miles 

 and its general width is 2 miles and less. It is impossible to state any 

 accurate dimensions because of the dissected condition, in consequence 

 of which long promontories run out from the main border and leave 

 intervening gulches. A very large promontor}^ extends northeast and 

 southwest to the west of the re-entrant angle in which is Fifteen-mile 

 spring. 



The mesa is built up of a complex of at least two and probably three 

 distinguishable flows and there may be a number of others. These are 

 recognizable because they appear one over the other, and in a few places 

 show evident contacts. At the head of the Fifteen-mile Spring gulch we 

 believe that we can distinguish a lower flow of light colored pumiceous 

 lava, on top of which rests a later one of dark, vesicular lava. The con- 

 tact is not specially sharp, but appears to be masked b}^ a bed of badly 

 weathered volcanic breccia. The under flow extends far to the northeast 

 beyond the escarpment of the upper one, and forms the northwest en- 

 closing wall of the Fifteen-mile Spring gulch. The two flows seem to 

 have followed one another at comparatively brief intervals, since the 

 observable, secular weathering of the undermost sheet is slight. Still, too 

 much importance must not be attached to this matter, because the 

 leucite rock resists weathering notably well. The separate flows also 

 appear in distinct terraces and by this feature indicate their relation- 

 ships. Single sheets certainly extend several miles and of thickness not 

 greater than from 30 to 50 feet. Some portion of the top has no doubt 

 been removed in the long course of geological time, and has thus caused 

 the sheet to appear today with less than its original thickness. The 

 upper surface is usually fairly smooth and resembles a huge tessellated 

 pavement, because of the joints ; again it is rough and ridge-like, because 

 of cross-depressions. Small faults are quite probably the cause of some 

 of the irregularities. The sheets were doubtless nearly horizontal when 

 they were originally poured out, but they show evidence of tilting to one 

 who sights with a hand level. From the summit of cone number 2 it is 

 evident that the lava flow slopes upward to the south, so that its south- 

 ern rim is as high as the cone itself. This would involve a rise of about 



XLV— Bum,. Geoi,. Soc. Am., Vol. U, 1902 



