322 D. VV. JOHNSON VOLCANIC NECKS OF MT. TAYLOR REGION 



existed, while vertical contacts are visible about most of the necks and 

 sometimes almost completely around them. 



The structure of the buttes, as already described, indicates their func- 

 tion as conduits leading to some surface higher up, and is quite distinct 

 from the normal structure of flows. The stringers and dikes, which 

 sometimes run from the buttes into the surrounding sediments, are not 

 normal accom^ianiments of surface flows. Sections of the buttes in the 

 sides of Mount Taylor mesa exclude the flow-remnant hypothesis. 



O. AS VOLCANIC KECKS 



If the buttes of the Mount Taylor region are to be regarded as true 

 volcanic necks, representing the material which remained in tubes or 

 chimneys connecting with vents at some overlying surface, they ought to 

 show certain critical features. 



Being revealed only as a result of the removal of overlying cone and 

 surface flows, volcanic necks ought to be found in the areas subjected to 

 great erosion. They are essentially erosion features. We should expect 

 them to occur in the valley lowlands and not on the adjacent highlands, 

 especially if the highlands represented the former surface and were cov- 

 ered with portions of surface cones and flows not yet destroj^ed by ero- 

 sion. Neither should the necks rise above the former level of the lava 

 cap, as indicated Ity associated- mesa remnants, unless, as might occasion- 

 filly happen, tlie upper continuation of tlie neck into the cone wei-e more 

 or less preserved. 'J'he buttes of the Mount Taylor region occur only in 

 the valley lowlands produced by erosion since the volcanic period, and 

 seldom rise above the former level of the lava cap as indicated by the 

 elevation of Mount Taylor and Prieta mesas. The exceptions noted have 

 features which indicate that their upper continuation into the overlying 

 cone are to some extent preserved. 



Volcanic necks should often show evidence of repeated eru})tions, both 

 explosive and quiet. The dift'erent tongues or stringers of lava would 

 show irregular columnar structure where various eruptions of different 

 dates were represented ; but where a single lava flow welled up to tlie sur- 

 face and then solidified, the lava remaining in the conduit might show 

 quite regular columnar structure. That part of the lava cooling deep in 

 the conduit would have the surrounding sedimentary walls as the cooling 

 surface, and the columns would therefore tend to develop a radial ar- 

 rangement; but that portion of the lava nearer the top of the conduit 

 would find a much more effective cooling surface in the free air above the 

 crater, and the columns developing perpendicular to this cooling surface 

 Avould be vertical. Passing downward,' a point would finally be reached 



