FORMER COXXECTIOXS OF THE MESAS 333 



have probably been broken apart by erosion. Thus Zirkel mesa and 

 Emmons mesa were no doubt originally one flow, since they are now 

 separated by a relativel}^ narrow gulch and front each other with escarp- 

 ments that match for nearlj^ 2 miles. That, however, the others were 

 connected is improbable. As between Zirkel mesa and Orenda mesa the 

 interval of erosion is so great as to make the supposition that they ever 

 were united altogether improbable. Of all the rest much the same holds 

 true. The intervening gaps and stratigraphical positions preclude the 

 supposition. On the contrary, we are justified in picturing in our imagi- 

 nations a landscape which erosion had brought somewhat toward its 

 present conditions. In the midst of this a number of eruptive centers 

 broke out, now represented by 22 separate exposures which remain. 



There appears to have been some tilting and disturbance of the sur- 

 face flows since their eruption, because Zirkel mesa no longer lies flat. 

 Its southern edge is as high as the summits of the cones at its northern 

 side and corresponds to the dip of the underlying Laramie. The flows 

 also rise toward Emmons mesa on the west. These are positions which 

 a molten surface flow could not well have assumed unless it had flowed 

 down hill from sources outside the present exposures. In the latter 

 event we ought to find the dikes or vents wdiich fed them, and of these 

 there is no trace. 



There has also j^robably been some minor faulting, to which may be 

 attributed the small escarpments and gulches on the western portion 

 of Zirkel mesa. The break of a fault ma}^ have located and aided the 

 formation of the gulch now existing between Zirkel and Emmons mesas. 

 Faults may be demonstrated in the Laramie outside the flows of leucitic 

 lavas. Thus one is ver}' pronounced at the western side of the entrance* 

 to the valley of Fifteen-mile spring on the north side of Zirkel mesa. 



As already noted, a fault has certainl}^ broken and sheeted the rock 

 of the Boars tusk, and the surface of Steamboat mesa is very irregular, 

 being broken by gulches and hillocks, which are probably not all due 

 to erosion pure and simple. There are so few springs that to subter- 

 ranean circulations and consequent caving it is not easy to attribute 

 much efficiency. 



In some of the smaller mesas disturbances seem generally to have 

 failed. Thus on Cross mesa the lava sheet is a dead level, so far as the 

 eye can detect, for at least a mile west of the cone. 



The Cones 



The cones present to the observer some of the most striking features 

 of the region. They are of two kinds, fragmental or cinder cones and 



