STRUCTURAL DETAILS 319 



tions. At the nortliern end of Mount Taylor mesa the lava seems to 

 show distinct bedding with a slight dip to the south, while just north is 

 one of the buttes. The features strongly suggest a neck which was a 

 conduit to a volcano, and the lava flows sloping away from that volcano, 

 the cone itself having been removed by erosion. 



Button has described in some detail a number of the buttes in other 

 parts of the region. According to his descriptions, vertical columnar 

 structure is not uncommon; evidence of repeated eruptions from the 

 same vent is abundant; the buttes are often elongated rather than cylin- 

 drical, and the long axis may be prolonged in both directions by dikes. 

 No evidence of recent flows is found in connection with the buttes. It 

 appears, therefore, that there is a general uniformity of essential features 

 in the buttes of different parts of the region. In minor details there is 

 variety, some buttes being large, others small, with every gradation in 

 size between the two extremes. Some are composed almost wholly of 

 agglomerate, some almost entirely of basaltic lava, while others are inter- 

 mediate in composition; some of the buttes appear nearly circular, others 

 are more elongated, usually in a north-south direction; dikes are visible 

 connecting two or more of the buttes, or extending a shorter distance 

 north and south of a single butte, or appear to be wholly absent. But, 

 as regards those features critically important in the present discussion, 

 tiie relations are uniform and simple. The buttes are located in the low- 

 lands produced by erosion, are not associated with lavas more recent 

 than tlie main great lava cap which formerly extended out over them, 

 have evidently served as vents for repeated eruptions, and show every 

 gradation from buttes well stripped of enclosing sediments, through those 

 showing more or less of the sediments making vertical contacts with the 

 igneous rock, to examples just beginning to be exhumed from the walls 

 of the mesa and which still connect with overlying cones and lava flows. 



Possible Interpretations 

 a. as remnants of laccoliths or sills 



Can the buttes of the Mount Taylor region be interpreted as remnants 

 of laccoliths or sills, all the overlying beds having been removed by ero- 

 sion, as well as all the intrusive mass except a small more or less cylin- 

 drical shaft ? Such an origin has been announced for the Devils tower, in 

 Wyoming, partly because of its vertical columnar structure and undis- 

 turbed associated sediments. We have seen that many of the Mount 

 Taylor buttes show vertical columnar structure, though never so perfect 

 as that in the Devils tower, while practically all of them are associated 



XXVII — Bull. Gbol. Soc. Am., Vol. 18, 1906 



